feat: getmud UC1

[yucongalicechen]

Problem

Proposed solution

Implement this in a library format: return mu or D for given muD (we can reuse functions from computing theoretical muD).

[yucongalicechen]

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

UC2: {sjb: I doubt if this one will ever be used, but we could check with Till/Pascal?}

1. User has sample information, energy, target diameter and muD, and wants to know the sample mass density.
2. As UC1.2-3
3. User gets the target sample mass density.

UC3:

1. User specifies sample composition and energy.
2. As 2-3 in UC1.
3. User receives (multiple) recommendations for mu depending on phase
   (lpp defaults packing fraction to 0.5 and 1 and computes using the function in diffpyutils)

UC4:

1. as UC1 but user supplies packing fraction and no density

@sbillinge could you check if the UCs look good to you? Thanks.
@yucongalicechen I simply edited these. I hope that is ok because it is probably hard to get back the earlier version....

[yucongalicechen]

Closing this as UC1 is not needed - the user probably already knows the diameter if they have information on mass density from measuring. The other UCs for getmuD module are tracked in #181.
@sbillinge Since this is the last issue on 0.2.1 pre-release, shall we release it first? The remaining getmuD UCs are tracked in the next release. If we want to implement some getmuD functionality to see how it looks, I think we can start with UC3-4 in #181, as they are easier.

[sbillinge]

We can release it. The reason would be to have people test it. But I think we are changing the interface are we not? So we probably don't want to do that. Let's maybe revisit the interface and the docs. If you bring the code pu to the latest scikit-package standards I think we can preview the docs more easily....