Speed up SICOR for EnMAP
There seems to be a lot of room for speeding up the entire processing in SICOR for EnMAP.
- make_ac_enmap() takes around 5 min per detector but runs in singleprocessing
- the water vapour first guess retrieval takes around 2 min
- multiprocessing is disabled for Windows (#42)
- Mac OS uses 'spawn' as default multiprocessing backend which might be slow with globals.
- consider switching from multiprocessing to joblib which would make the implementation much easier
- there are a lot of nested for-loops in the code, many of them can for sure be optimized or vectorized