Provide rhoc from theta_grid

[David Dickinson]

Theta grid is supposed to be the module representing the geometry and the theta grid – most parts of the code shouldn’t use geometry directly. Dist fn uses rhoc from geometry as theta_grid doesn’t currently provide this. In fact, theta_grid_params should always hold the true rhoc value and this is then copied into geometry.

We should probably store rhoc in the main theta_grid module so dist_fn no longer uses geometry directly.