Allow different superposition models for wake and blockage + add blockage to flowmap with weightedSum (was missing before)

fixing "Using a DataArray object to construct a variable is ambiguous, please extract the data using the .data property" issue introduced in xarray 0.19.0

- fix ct scaling (causing issues and wrong levels)
- add PowerCtGenericTIRho
- improved error message when interpolating out of bounds with PowerCtNDTabular

fix #44

Add option to calculate AEP as described in the European Wind Atlas, page 95. This method calculate the power x weibull_weight using linear instead of constant power segments

Before the result of the GridInterpolator was transposed for 1 additional dimesion and mixed for more. This has been fixed