Remove out of spec requirement that calibration explicit values be
sorted and not empty.
Improve convert_unit to deal with non SI units (angstrom and degrees).
Dictionary returned by orphan conditions is now a Conditions() dict.
Prevent error when converting None value.
Add method to get calibration from WDS detector in different quantities
(energy, wavelength and unit).
Add physics module for simple physical equations and constants.
Allow different calibration to be passed to get_xdata(...),
get_xlabel(...), etc.