splitting out some of the material property functions

[chrishavlin]

Splitting out the material property functions to make them more useful (thermal conductivity, thermal expansion, adiabatic compression, specific heat).

Still to do:

• [x] add some tests for the individual functions
• [x] think about reorganizing files -- maybe move these to a vbrCore subdirectory?
• [x] update release_notes

[chrishavlin]

If reorganizing where the files live, should move the solidus function in addition to those mentioned above.

[chrishavlin]

files reorganized, also added a few helper functions for calculating adiabatic quantities (adiabatic gradient dTdz, adiabatic pressure gradient dPdT) and an empircal function for calculating olivine density vs Pressure.

[chrishavlin]

functions using FracFo can now accept FracFo as a vector

[chrishavlin]

expanded this to include the matlab-only test ability, so this Closes #41

[chrishavlin]

and now Closes #47 as well

[chrishavlin]

aaaand updated the readme.