ENH: Add from_annot and/or from_seq / from_bbox classmethods to format classes?

[NickleDave]

One can make a format from a sequence by unpacking keywords one-by-one, but why not provide a classmethod as a convenience method that does this on a per-class basis? (Since some will require diff't keywords depending on their attributes)

For seq formats, the from_annot method can just check that it has a seq attribute and then call from_seq with that attribute. Same for a bbox format, except we check for a bbox attribute.

This also makes the use case easier where one wants to convert back to a method -- e.g., I have a Sequence (or Annotation with a seq). As in #207, I could write NotMat.from_seq(seq).to_notmat(notmat_path).