Add missing checkpoints for most multi-entrance Metroids

[dyceron]

Did not test most of these since it requires randovania/mercury-engine-data-structures#88 to be merged first, otherwise rooms break

• Added checkpoints to all but two multi-room Metroids (skipped ones connect to an item room so checkpoint isn't that important)
• Renamed Gamma 2b checkpoints to match the vanilla naming scheme

[ThanatosGit]

Looks like most, but not all, methods follow the same pattern. Could maybe generalize it later.