PowerSystems importer or Data layer

[coroa]

Either import a PowerSystems.jl model into the DictData data container (#15) or provide a separate data layer.

The main tricky bit is the same as with the PyPSA models, on how to split the generator definitions into separate classes and how to name them.

Maybe one can abstract that problem away?

PowerSystems will then allow running their examples from https://github.com/NREL-SIIP/Examples , and provide a tried and tested PSS/E raw and MATPOWER case file importer.

Makes a huge contribution to examples for #9 .