Reduce memory consumption of All2AllIterative with NOJDeficit + add test of memory usage and memory leak

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

implement support for i-dependent WS and WD in site and wt-dependent WS and WD kwarg to WindFarmModel.__call__

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