Implement OpticalFlow transform

[JackKelly]

See notebooks/design.ipynb for ideas.

Probably needs to be single process, because the daemonic worker processes can't spawn processes.

See this notebook for my last attempt at optical flow (using multiple processes): https://github.com/openclimatefix/predict_pv_yield_2/blob/master/notebooks/16_maxpool.ipynb

[JackKelly]

Duplicate of #96