Implement write_eigenfunc in new diagnostics

Replaces PR #45 (updating it for 8.0.4-RC + python)

Addresses issue #25

Implement writing the eigenfunction (`write_eigenfunc`) in the new diagnostics. This moves write_eigenfunc from diagnostics_final_routines (a list of functions that haven't been upgraded) to diagnostics_fields. As suggested by @rnumata , this also removes the call to do_write_eigenfunc, which should fix #25.

This also adds tests to check that runname.eigenfunc, and the netcdf variables phase, phi_norm, apar_norm and bpar_norm are the same for old and new diagnostics.