Rename tedana.model to tedana.metrics

[tsalo]

Summary

With #273 we'll be moving statistics-related functions out of tedana.model.fit, and so that module will become purely about metric calculation. This rename was proposed by @emdupre in her review of #266, but will be done separately to reduce PR bloat.

Next Steps

1. Rename tedana/model/ to tedana/metrics/.
2. Update all imports throughout package, including tests.
3. Update API documentation.

[jbteves]

@tsalo have you started on this already? If not I'll try to take a crack at it.

[tsalo]

I was planning to wait until after #266 was merged, and then never circled back to it after that happened. It would be awesome if you could do it.

[jbteves]

Cool, gave it a stab in #325 and requested your review.