Add +restart_repro variant

[aidanheerdegen]

Restart reproducibility variant has been added to the MOM5 SPD:

ACCESS-NRI/spack-packages#67

To address lack of restart reproducibility.

This variant should be added to ACCESS-OM2 via a PR.

This will allow for this fix to be tested, and for performance testing to be done on a wider range of configurations.

Once tests pass and performance testing has been signed off this can be merged.

[aidanheerdegen]

Made the +restart_repro variant default True for mom5 instead

ACCESS-NRI/spack-packages#83