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

Reduce memory consumption in PropagateDownwind by doing inplace flattening of dw_iil, hcw_iil and dh_iil. Note that this may have side-effects in the parent functions

fix "outside area" error occurring when making a flow map with XRSite (which fails because local wind at (0,0) is requested

before default rotor center was shared among wind farm models, i.e. wakedeficit model of last instance was applied to all instances