USE CASES - get mud

[yucongalicechen]

          OK, this is great now we have the UCs.  I think we are doing UC1 and UC3 and UC4.

UC1 is composition+energy+rho -> diameter
UC3 is comp + E -> mu (fully dense and 50% packing fraction)

I think that we probably want:
F1: comp + E + density -> mu
F2: comp + E -> estimated mu after looking up a CIF

Then we can satisfy
UC1 as muD/F1(comp,E,density)
UC4 as muD/F2(comp,E)
F2 satisfies UC3

Please could you close this and make one PR for each of those functions?

Do we also need a command line app that the user runs to get this info? Let's put that on a separate PR, with the docs too.

It is already 3:20 so I guess we won't get this merged by 5. Shall we push off that meeting until tomorrow?

Originally posted by @sbillinge in #175 (review)

[sbillinge]

I am not sure what this issue is? Is it just to capture the UCs? In that case give it a title something like USE CASES - get mud. Then we know it is not a task that needs to be completed but kind of a documentation record.

[yucongalicechen]

I am not sure what this issue is? Is it just to capture the UCs? In that case give it a title something like USE CASES - get mud. Then we know it is not a task that needs to be completed but kind of a documentation record.

^edited.