Remove transformer attribute from bases, transformer load bug fixes

[shanemcq18]

• Fixed a bug where the SnapshotTransformer class would not persist correctly when saving to an external file and loading later.
• API Change: Basis and ROM classes now have compress()/decompress() methods instead of encode()/decode() methods. A few versions ago these were project()/reconstruct(), I promise this is the last name change.
• minor API Change: Basis classes used to accept a transformer object at instantiation so that basis.compress() would do the transformation and the dimension reduction. This lead to some confusing bugs in practice (and unnecessarily complicated code), so removed this feature.

[shanemcq18]

Documentation build failures are due to an issue in newer versions of Jupyter Book. Setting a cap on the version to avoid this (should remove it later when JB is fixed).