v1.0: Move to `beamphysics` package name and deprecate `pmd_beamphysics` import

[ken-lauer]

After the other v1.0 milestones have been reached, including:

• Finishing and merging WIP/ENH: wavefront class #68
• (Others?)

openPMD-beamphysics will:

• Migrate the package to be importable as simply beamphysics.
• Deprecate the current pmd_beamphysics import.
  • This will show up as a deprecation warning until v2.0
  • After v2.0, the import will be removed entirely