Bug in filtering.plot_postprocessed_in_ws_power_curve

[Bartdoekemeijer]

Apologies for the short issue report, but wanted to flag this rather than hardcode it in my version.

Using more recent versions of matplotlib (> 3.9), legendHandles was replaced by legend_handles. See https://stackoverflow.com/questions/12848808/set-legend-symbol-opacity.

Hence, we must replace legendHandles with legend_handles in this line of code: https://github.com/NREL/flasc/blob/686c62d9b757f598b45d775c604bb773cb7bf7e8/flasc/data_processing/filtering.py#L899