Bugfix/ensure restart file includes explicit source

• Don't allocated explicit source arrays if not needed

• Move explicit source array declaration to dist_fn_arrays

  Remove dimensional comment

• Add explicit source terms to restart file and restore if present.

  Decides if we should try to store/restore gexp_1/2/3 if the arrays are allocated at the point of calling gs2_save_for_restart/restore_restart.

  Currently doesn't abort if we can't restore these arrays in order to preserve backwards compatibility with existing restart files. We should consider making this abort once the feature has been in release from a version or two.

• Save extra timestep history to restart file

• Store code_dt rather than passed time step.

  No change to save_for_restart interface yet to reflect this.

• Update init_dt and init_delt to restore/set the full timestep history rather than a single value

Ensures the full explicit source term history is stored/restored when saving/loading a restart file. Also store the full time step history. This is restored if delt_option = 'check_restart' as usual.

This is a partial replacement for PR #108