version 3.3.0

Improvements:

• add ZipStorage, support loading SBT databases from storage; .sbt.zip extensions. (#648)
• Replace khmer.Nodegraph with rust nodegraph; ~5x speedup of SBT search & gather. (#799)

Bugs:

• Document and (lightly) fix the LCA_Database API. (#966)
• Fix bug when using Python 3.5 and before; refactor LCA_Database tests (#962)

Documentation:

• Document gather abund tests a bit better; minor refactoring (#886)
• Improve lca index error (#963)