Rename Particle Species

**Is your feature request related to a problem? If so, please describe.**
In https://github.com/openPMD/openPMD-standard/pull/183#discussion_r409111243, @DavidSagan proposed to rename in the base standard the term "Particle Species".

**Describe the solution you'd like**
Proposed: Particle Group

**Describe alternatives you've considered**
I tried reformatting the base standard and found it reads confusing to use "particle group" since we also define the term "group" for "directory-like" groupings in files.

Maybe another term such as "Particle Collection" would work?

**Additional context**
Used as naming in `BeamPhysics` extension: https://github.com/openPMD/openPMD-standard/pull/183