ENH: Rewrite formats as sub-package of classes, rewrite API

[NickleDave]

Rewrites formats as classes, and closes many related issues. Big step towards version 4.0 as described in #146

• [x] Rewrite formats as classes, fix #99
• [x] rewrite API -- fixes #120
• [x] add formats bass class -- fixes #105
• [x] write annotations as attrs classes -- fixes #99
  • [x] refactor existing formats in process
  • [x] refactor generic-seq format, use pandas -- fixes #63
  • [x] fixes #154
  • [x] fixes #134
• [x] add formats sub-package -- fixes #109
• [x] change how annotation formats are listed -- fixes #92
• [x] remove entry points -- fixes #94