mu T calculator (UC1)

[yucongalicechen]

Problem

Implemenet functions to accomplish this user case:

1. user places sample on diffractometer
2. diffractometer determines weight and dimensions of the sample
3. diffractometer shares this information with diffpy.labpdfproc
4. diffpy.labpdfproc computes muD

(See more discussions in #126 and diffpy/diffpy.utils#45)

Proposed solution