Thermodynamics

I've added a module which calculates density offline using the ICON equation of state. The code ingests Xarray objects so things are automatically parallelised with Dask for large datasets. In the future someone might want to have a play optimising certain functions with numba, C or Fortran, but I think this is a good start. The code is based on the ICON thermodynamics routines and there's some tests to go with it. The code works for z-star and ordinary vertical levels. I've also tried to make things flexible enough for someone to implement other equations of state in the future should they wish.