capacityconst

major:

--- added a capacity constraint for maximum wind farm installed capacity. The constraint code structure complies with framework. This constraint is only used as penalty because the turbine types are determined by the "integer optimization level".

--- added tests (no partials required as far as I see the usage, the only optimizer input is turbine type.)

minor changes:

--- have changed some of 'type' and 'x' in spacing and plotting components to topfarm.x_key,topfarm.type_key etc for consistency.

After this merge request resolved i will add a new example for turbine type optimizaiton with capacity constraint showing it works. This example needed some lines change in wind_turbine.py as well. Not sure where i should add this example: in examples/docs or somewhere else ?