Change NXdata scaling_factor to refer to "plotted" data
Change NXdata to refer to "corrected" data, in addition to "physical" data, since it describes units of photons

This resolves ambiguity if there is more than one signal

For NXmx specify data_scaling_factor and data_offset since the field data is named in the NXdata group

- NXmx: fully define pedestal, bias, and gain and how they relate to offset and scaling_factor
- Add reserved suffixes _scaling_factor and _offset
- NXdata: define better how the stored values in FIELDNAME are converted to physical values
- NXdata: more clarification on how scaling_factor and offset without FIELDNAME are ambiguous

- NX_FLOAT -> NX_NUMBER for scaling_factor and offset
- Sync language with number #1396 for the FIELDNAME convention