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 #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

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

add function to create XRSite from simulation_result.flow_box() allowing precalculated sites including effects of neighbouring farms