muD lookup table

[sbillinge]

expected behavior:
user specifies sample composition. Given known wavelength the program computes a best effort muD. at this point make muD an optional argument. if muD and composition are given muD = args.mud, but composition is written to metadata.

If possible reuse other code for parsing/handling composition. We are doing this in getx3 already. This cannot be a dependency because it is proprietary, so maybe pull that code out into diffpy.utils?