New features:
- gravity profile in refstate
- isocolors command line option to specify isoline colors
- plot_iso passes kwargs to the contour matplotlib function
- isolines are colored according to field.cmap if no isocolors
  is specified

Documentation:
- refstate module API documentation

Bugfix:
- magma_oceans_mode radial coordinates did not work since commit
  9506170d, this is now fixed