Add dipole mapping functions

[klaundal]

We should add functions for mapping different quantities along dipole magnetic field lines. This is not complicated to do with the apex basevectors, which we already calculate, but it would be good to have functions to do it explicitly.

[jpreistad]

I've added a function for mapping the perp part of the velocity vector down to a specified height, in the field_line_mapping branch. My testing was not very extensive, but what I tried out seemed to produce reasonable results.

Are we ready to merge this into main? Another reason to merge this is that the E3DSECS package needs the 'get_apex_base_vectors_geo()' function, currently only part of field_line_mapping branch.