Add new Reservoir node

[jetuk]

Add a new reservoir node with the following features:

• Evaporation and rainfall parameters (typically defined as L (e.g. mm))
• Bathymetry (i.e. area and level) data to construct parameters that determine current area and level from volume (see also Add parameter name spaces #179 )
  • Have evaporation and rainfall flow calculated from above.
  • Optionally allow rainfall to always use the maximum area of the reservoir (i.e. all rainfall runs off into the reservoir regardless of current volume)
• Add optional output slots
  • Spill
  • Compensation
  • Losses (Output)

Agreed to not include abstraction or hydropower nodes initially.

[jetuk]

#256