Add interface to RESkit power curve model

RESkit, Severin Ryberg's atlite :), has three very interesting features that one should evaluate, whether they can be adopted or reused:

1. A synthetic power curve model that is defined in reskit/wind/core/power_curve.py, which generates a synthetic power curve only on the basis of a specific power rating, ie. kW/m^2 (capacity rating over rotor area) or capacity and rotor diam equivalently, maybe one can just reuse or sub-class that code with a small modification to make the simulate method work with xarray/dask.
2. An economic capital cost scaling model around the turbine parameters hub height, rotor diameter and capacity (and foundation type for offshore), which is defined in reskit/wind/economic/{onshore_cost_model.py,offshore_cost_model.py}. This maybe is a function that would be called from PyPSA-Eur directly.

Together those two features would allow having a range of different turbines or the possibility to have a weak wind turbine as a second choice.

Start by reading the referenced RESKit files: reskit/wind/core/power_curve.py, reskit/wind/economic/onshore_cost_model.py, and reskit/wind/economic/offshore_cost_model.py. Determine how the power curve could work with xarray/dask and whether the cost models can be reused; done means defining an interface that supports multiple turbine choices, including a weak-wind option.