Restructure and documentation

[christiaanjs]

Proposed structure:

• treeflow
  • tree - data structure (rooted/unrooted?), topologies, parsing, writing
  • traversal - pre/postorder traversals with TensorArray
  • distributions
  • phylo_likelihood - important submodule?
  • tree_distributions - wrappers/utils, priors
  • clock_model - conjugate models, VI approximations
  • bito - bindings for bito/beagle
• treeflow_pipeline - (existing name) - treeflow-cli? - same or different repo?
  • vi
  • map