docs: comments and critical error logging

Added comments and critical error logging i `indexStore` when creating
sub-directory per Index.
enable parallel indexing with sortStore

Removed a `NotImplementedError` when setting up the sortStore
BerkeleyDB connections in multiprocessing sessions.
docs: more comments and logging for `indexStore`

Additional comments and error logging when committing an `Index` to
an `IndexStore`.
bugfix: Index vectors built in parallel

closes #69
bugfix: sortStore and vectors in parallel

Fixed advanced Index settings, `sortStore` and `vectors` when indexing
in parallel.

Merge branch 'feature/multiprocess' into develop