Small modifications to clarify NXmpes

[lukaspie]

• clarify that the explicit axis names are not exhaustive and other names are allowed
• rename all environments in NXmpes/NXsample to *_env to distinguish them from the same NX_FLOAT concepts in the base class NXsample
• add the (not recommended) possibility to write a float value in these environments if there are no actuators/sensors
• allow multiple of pressure_gaugeID(NXsensor) on NXinstrument -> in preparation for a NXnapxps appdef
• remove changes to final_polarization and incident_polarization from NXsample

@rettigl @domna this bundles all of the changes we were discussing in the last few days, also with respect to the requests from Stefan and Uwe. My plan is to first merge this and then rebase the mpes-liquid branch and finally make the necessary changes on NXmpes_liquid there.

[domna]

Only one small comment on the incident/final_polarization/@units. Otherwise LGTM