Mixin updates

• Hook system implemented for mixins to use, and some now do (position and cache)
• MSA functionality is not a bit more flexible. Instead of just being passed to fit, they are now associated with sequence eg. sequence.msa attribute. This allows us to distinguish models that 1. have a wt and associated msa 2. are fit to an alignment not necessarily associated with a wt and 3. models that can operate on many different msas.