Base repair amount on ratio rather than fixed increment

[djwhitt]

Elite Command heals half damage with each repair. That seems like it might be overly generous, but repairing based on a ratio rather than a fixed amount seems like the right approach. See #59 for more background.

[tbeddy]

Healing half damage sounds fine to me if it's restricted to bases.
Would the :repair value for each unit type (currently 1 for all of them) only be for units that can field repair, or would that fixed amount also be scrapped in favor of a ratio approach?