Add solidus and liquidus as a function of pressure

[jrenaud90]

This will be easy to do for silicates (see the equations 21 and 22 in Shoji & Kurita 2014).

Run these equations at layer initialization (TidalPy does not allow for a changing pressure at the moment, so these do not need to be recalculated after the planet is built).

Now phase change for other materials, namely ice, will be a much bigger challenge and deserves its own issue.