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

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