muD calculator for z-scan files

[yucongalicechen]

move mud_calculator.py from labpdfproc to here

Check https://github.com/diffpy/diffpy.labpdfproc/blob/main/src/diffpy/labpdfproc/mud_calculator.py
and https://github.com/diffpy/diffpy.labpdfproc/blob/main/tests/test_mud_calculator.py.

[sbillinge]

btw @bobleesj we can put this switch into the 3.7.0 release if we want. It shouldn't hold up 3.6.0

[bobleesj]

yeah @yucongalicechen 3.7.0 would be good..

[bobleesj]

@sbillinge