Python: useful error when initialize_warpx not called before creating ParticleContainerWrapper

[archermarx]

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

[dpgrote]

I would suggest handling this in a different way. Now, the issue is that the libwarpx.warpx is accessed in the __init__ routine. A change would be to only access libwarpx.warpx when it is needed. Perhaps make particle_container a property that gets the underlying particle container when needed. The user can then create the ParticleContainerWrapper at any time. The message this PR adds would then go in the particle_container property get method. This is how the field wrappers are setup.

[archermarx]

Got it, thanks for the feedback!