Add more possible values to NXsensor/measurement

[jkotan]

at DESY with SECoP@HMC we are trying to improve our description of sample environment.
Therefore It would be nice to add "humidity", "viscosity" and "concentration" as a possible values of NXsensor/measurement.
They exist in the SECoP framework but we don't have them as a possible values of NXsensor/measurement

[g-guenther]

I propose to close this issue since it is included in PR #1574.