+Split ALE_main into ALE_regrid & ALE_remap calls

  Replaced ALE_main with ALE_regrid and calls to ALE_remap_tracers and
ALE_remap_velocities from step_MOM_thermo.  Also eliminated ALE_main_offline and
ALE_offline_tracer_final, as they can now be replaced with a calls to ALE_regrid
and a call to ALE_remap_tracers.  Also added unit descriptions to a number of
comments describing variables in MOM_ALE.F90.  All answers are bitwise
identical, but there are 3 new publicly visible subroutines (ALE_regrid,
ALE_remap_tracers, and ALE_remap_velocities) and three previous interfaces
(ALE_main, ALE_main_offline and ALE_offline_tracer_final) have been eliminated.