feat: getmuD UC3-4

[yucongalicechen]

Problem

The UCs for getmuD (pasted from #166):
UC1:

1. User has sample composition, energy, and mass density and a target muD in mind, and wants to check the diameter.
2. User provides this information labpdfproc.getmud (lpp.gm)
3. lpp returns the target diameter

UC3:

1. User specifies sample composition and energy.
2. User provides this information labpdfproc.getmud (lpp.gm)
3. lpp.gm computes mu for fully dense sample
4. lpp.gm returns mu and mu/2

UC4:

1. User has sample composition, energy, and packing fraction and a target muD in mind, and wants to check the diameter.
2. User provides this information labpdfproc.getmud (lpp.gm)
3. lpp returns the target diameter

For this issue we want to implement UC3-4 and we should have CLI apps for each UC.

Proposed solution

For UC3:
lpp defaults packing fraction to 0.5 and 1 and computes using the function in diffpyutils), so the code can be something like:

def estimate_mu(sample_composition, energy):
    mu1 = compute_mu_using_xraydb(sample_composition, energy, packing_fraction=0.5)
    mu2 = compute_mu_using_xraydb(sample_composition, energy, packing_fraction=1)
    return mu1, mu2

Implement this after we complete diffpy/diffpy.utils#331.

For UC4, we just need to add the tests (check test_estimate_diameter).

[sbillinge]

looks good. I slightly modified UC3 based on our conversation. We have to decide an algorithm for computing mu when there are more than one phases returned by the database. There are different ways of doing it, we can discuss.