Fix RXD failures on ARM

[JCGoran]

In #3177 we skipped some of the CVODE-related tests for RXD on ARM platforms.
To reproduce in the CI on ARM-based MacOS, here's a Github Actions file:

name: Build MacOS Wheels

on:
  push:
    branches:
      - master
  pull_request:
    branches:
      - master

# do not cancel so we can see results
concurrency:
  cancel-in-progress: false
  group: ${{ github.workflow }}-${{ github.ref }}

jobs:
  build-test-deploy:
    name: Build MacOS Wheels
    runs-on: ${{ matrix.os }}
    timeout-minutes: 60
    strategy:
      matrix:
        os: [macos-14]
        python-version: ['3.10']

    steps:
      - name: Check out code
        uses: actions/checkout@v4

      - uses: actions/setup-python@v5
        with:
          python-version: ${{ matrix.python-version }}

      - name: Install OSX System Dependencies
        run: |
          brew install --cask xquartz
          brew install flex bison mpich
          brew unlink mpich && brew install openmpi
          cmake --version
          # Uninstall libomp for compatibility with issue #817
          brew uninstall --ignore-dependencies libomp || echo "libomp doesn't exist"
          brew install cmake
          echo "$(brew --prefix)/opt/cmake/bin" >> $GITHUB_PATH
          echo "$(brew --prefix)/opt/flex/bin:$(brew --prefix)/opt/bison/bin" >> $GITHUB_PATH

      - name: Install readline
        run: |
          sudo mkdir -p /opt/nrnwheel/$(uname -m)
          sudo bash packaging/python/build_static_readline_osx.bash

      - name: Build MacOS Wheel
        run: |
          packaging/python/build_wheels.bash osx ${{ matrix.python-version }} coreneuron

      - name: Test wheel
        run: |
          minor_version="$(python -c 'import sys;print(sys.version_info.minor)')"
          packaging/python/test_wheels.sh $(which python) wheelhouse/*cp3${minor_version}*.whl

      - name: Upload wheel files
        uses: actions/upload-artifact@v3
        with:
          name: macos-wheels-${{ matrix.python-version }}
          path: wheelhouse/*.whl

To reproduce on ARM-based Linux, since Github Actions does not support ARM-based Linux runners, one can do the following locally (I will use podman/docker commands interchangeably):

1. install podman or docker
2. (if not running on ARM-based MacOS) run docker run --rm --privileged docker.io/multiarch/qemu-user-static --reset -p yes to enable emulation of ARM on x86_64
3. make clean clone of NEURON: git clone https://github.com/neuronsimulator/nrn
4. run the following:

docker run --rm \
    -w /root/nrn \
    -v $PWD:/root/nrn \
    -e NEURON_NIGHTLY_TAG="" \
    -e NRN_NIGHTLY_UPLOAD="true" \
    -e NRN_RELEASE_UPLOAD="false" \
    -e SETUPTOOLS_SCM_PRETEND_VERSION="9.0.0" \
    --platform linux/arm64 \
    -e NRN_BUILD_FOR_UPLOAD=1 \
    'neuronsimulator/neuron_wheel:latest-gcc9-aarch64' \
    packaging/python/build_wheels.bash linux 3.10 coreneuron

This will put a NEURON wheel in the wheelhouse dir.

5. put the following contents in a file called Dockerfile:

FROM docker.io/ubuntu:22.04
RUN apt-get update && apt-get install -y python3-venv python3-pip
# you may need to tweak the below to match the exact filename of the wheel produced in step 4
COPY wheelhouse/NEURON-9.0.0-cp310-cp310-manylinux_2_17_aarch64.manylinux2014_aarch64.whl .
RUN python3 -m venv env
RUN env/bin/python -m pip install NEURON*.whl
RUN env/bin/python -c 'import neuron; neuron.test_rxd()'

6. run docker build --platform linux/arm64 -t neuron_reproducer .

In any case, the output is something like:

FAIL: test_ecs_diffusion_variable_step_coarse (neuron.tests.test_rxd.RxDTestCase.test_ecs_diffusion_variable_step_coarse)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/runner/work/nrn/nrn/nrn_test_venv_311/lib/python3.11/site-packages/neuron/tests/test_rxd.py", line 163, in test_ecs_diffusion_variable_step_coarse
    assert p.exitcode == 0
           ^^^^^^^^^^^^^^^
AssertionError

======================================================================
FAIL: test_ecs_diffusion_variable_step_fine (neuron.tests.test_rxd.RxDTestCase.test_ecs_diffusion_variable_step_fine)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/runner/work/nrn/nrn/nrn_test_venv_311/lib/python3.11/site-packages/neuron/tests/test_rxd.py", line 170, in test_ecs_diffusion_variable_step_fine
    assert p.exitcode == 0
           ^^^^^^^^^^^^^^^
AssertionError

Error size for the two appears to be:

FAIL: test_ecs_diffusion_variable_step_coarse (neuron.tests.test_rxd.RxDTestCase.test_ecs_diffusion_variable_step_coarse)
assert ecs_vec.sum() <= 1e-9, f"{ecs_vec.sum()=}"
AssertionError: ecs_vec.sum()=6.905884752939073e-09

FAIL: test_ecs_diffusion_variable_step_fine (neuron.tests.test_rxd.RxDTestCase.test_ecs_diffusion_variable_step_fine)
assert ecs_vec.sum() <= 1e-9, f"{ecs_vec.sum()=}"
AssertionError: ecs_vec.sum()=1.3189747141151287e-06