Dimension slicing utility

[pp-mo]

Found myself writing this just to use it.
It's pretty simple ...

slicedim_name = "time"
slicedim_len = 2
ncds.dimensions[slicedim_name].size = slicedim_len
for var in ncds.variables.values():
    if slicedim_name in var.dimensions:
        (i_slicedim,) = [
            i for i, name in enumerate(var.dimensions)
            if name == slicedim_name
        ]
        slices = [slice(None) for dim in var.dimensions]
        slices[i_slicedim] = slice(0, slicedim_len)
        var.data = var.data[tuple(slices)]

Probably wants to be slightly generalised.
Maybe control with a map dim_name: dim_slice
Then new dim length can be deduced for any-old input slice something like
dim.size = da.zeros(dim.size)[dim_slice].shape[0]
(using dask to avoid actually making an array)

N.B. goes in a 'utilities' space : cf #64