Add include_explicit_source_in_restart input flag

Adding to init_g_knobs for now as this is where at least one other flag controlling the restart files is included. In the long run we probably want to make gs2_save have a more typical structure, with it’s own namelist etc.

When the flag is .true. (default) we include the explicit source terms when saving/restoring restart files. If .false. then we don’t store the explicit source terms – this means the restart files will be smaller, but there will also be a small difference between extended runs and restarted runs.

Fixes #83 hopefully.