Unify functionality from Hydra-Pywr branches

[pmslavin]

This issue is to unify all of the features currently found across several branches of Pywr into a single definitive version, refactoring and/or reimplementing features as necessary in order to provide a clean and consistent code base.

This version should target recent releases of Pywr master only, and should support the current Hydra/HWI representation of parameters, recorders, tables and scenarios.

In addition, the facility should be added to decompose exported networks into multiple files, consisting of a base json file which references potentially many external datasets in a range of storage formats.