Pywr master compatibility

[pmslavin]

Changes made in pmslavin/pywr-dcopf.pywr_master_compat introduce compatibility with current Pywr master.

• Updates node types to support new Pywr loading procedure.
• Adds test for behaviour of initial_volume_pc arg of Battery node.
• Miscellaneous layout and node ctor validation changes.

[jetuk]

Thanks @pmslavin. I'll find time to review this. Reminds me this repo needs to have the CI setup. That might be a bit easier following the new pywr approach with Github Actions.