Add string descriptors to ParticleGroup

[ColwynGulliford]

The openPMD BeamPhysics extension has several optional str parameters that can be used label/describe a Particle Group: For example:

latticeElementName

type: Optional (string)
description: The name of the lattice element the particle are in. This only makes sense if all particles are in the same element. [Keep in mind that if particles are lost and the lost particles are also included in the file, not all particles may be in the same element.] Also see: locationInElement.

However, these are not implemented in the ParticleGroup python class.