Allow wind speeds to be corrected by a location-specific linear factor (e.g. from GWA)

[nworbmot]

We should bias correct ERA5's wind speeds using DTU's global wind atlas (GWA). This will raise wind speeds in e.g. onshore Germany and the Alps.

See a similar PR in PyPSA-Eur from @p-glaum.

Consensus was we should wait until ERA6 is published before taking this on, since ERA6 will have improved resolution.

Implementation

Allow some sort of matrix mask to be passed to convert_wind that pre-multiplies the wind speeds in convert.py. Perhaps provide functionality/example for GWA, or leave that to PyPSA-X models.