WIP: Topic/abstract storage

[breezykermo]

closes #124

This started as a PR to abstract out what mtriage uses to store media during runtime as an abstract storage layer, so that it could write media either to the local disk or to a remote storage such as S3. It has ended up as a serious rewrite of almost all internals.

• write conceptual description of abstract storage.
• write conceptual description of more robust etype implementation.
  • including support for custom etypes in lib/etypes
• detail refreshed docs/tutorial structure

• API for 'user-defined' functions changed to receive and return specific types.
• using pathlib for internal paths
• docs and tutorials have become much better structured, will submit another PR for this.