Python: useful error when initialize_warpx not called before creati…

…ng `ParticleContainerWrapper` (#5412)

Currently, WarpX is initialized when `sim.step` is called, or when the
user calls `initialize_warpx`. However, if the user tries to create a
`ParticleContainerWrapper` before this point, they get an error along
the following lines:

```
 File "/home/marksta/projects/warpx-ionization/picmi.py", line 185, in <module>
    sim.run()
  File "/home/marksta/projects/warpx-ionization/picmi.py", line 179, in run
    self.elec_wrapper = particle_containers.ParticleContainerWrapper(self.electrons.name)
  File "/home/marksta/.local/lib/python3.10/site-packages/pywarpx/particle_containers.py", line 29, in __init__
    mypc = libwarpx.warpx.multi_particle_container()
  File "/home/marksta/.local/lib/python3.10/site-packages/pywarpx/_libwarpx.py", line 46, in __getattr__
    return self.__getattribute__(attribute)
AttributeError: 'LibWarpX' object has no attribute 'warpx'
```
This is confusing. When I got this, I assumed I had maybe installed
WarpX wrong. I added a catch for this exception that re-raises it with
some additional context that should help direct the user to call
`initialize_warpx`

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Python/pywarpx/particle_containers.py

@@ -24,10 +24,21 @@ class ParticleContainerWrapper(object):
 
     def __init__(self, species_name):
         self.name = species_name
+        self._particle_container = None
+
+    @property
+    def particle_container(self):
+        if self._particle_container is None:
+            try:
+                mypc = libwarpx.warpx.multi_particle_container()
+                self._particle_container = mypc.get_particle_container_from_name(
+                    self.name
+                )
+            except AttributeError as e:
+                msg = "This is likely caused by attempting to access a ParticleContainerWrapper before initialize_warpx has been called"
+                raise AttributeError(msg) from e
 
-        # grab the desired particle container
-        mypc = libwarpx.warpx.multi_particle_container()
-        self.particle_container = mypc.get_particle_container_from_name(self.name)
+        return self._particle_container
 
     def add_particles(
         self,
@@ -758,7 +769,18 @@ class ParticleBoundaryBufferWrapper(object):
     """
 
     def __init__(self):
-        self.particle_buffer = libwarpx.warpx.get_particle_boundary_buffer()
+        self._particle_buffer = None
+
+    @property
+    def particle_buffer(self):
+        if self._particle_buffer is None:
+            try:
+                self._particle_buffer = libwarpx.warpx.get_particle_boundary_buffer()
+            except AttributeError as e:
+                msg = "This is likely caused by attempting to access a ParticleBoundaryBufferWrapper before initialize_warpx has been called"
+                raise AttributeError(msg) from e
+
+        return self._particle_buffer
 
     def get_particle_boundary_buffer_size(self, species_name, boundary, local=False):
         """