Refactor and document PyConf

This is a step towards migrating PyConf over to LHCb.

I've moved a few things around to try to avoid a massive core.py, but that didn't go too well because Algorithm, Tool, and DataHandle all depend on each other so can't be easily moved to separate files. For now I've kept Algorithm and Tool together in a components.py file and moved DataHandle and CompositeNode to a dataflow.py file. EverythingHandler now lives in environment.py. smart.py still exists but configurable and namespace are now exposed directly on PyConf (as I imagine these are the things most HLT-line modules will ever import from PyConf.

Also gone through and changed a few names, and added lots of docs (including doctests where possible).

Sorry the diff is so huge. I think in general the move is in the right direction but I'm not super happy with how things ended up being organised.

I can take care of rebasing the other MRs if we merge this. Includes !193 (merged).