New feature: add more spectral indices to utils/preprocess_data.py

[dbuscombe-usgs]

utils/preprocess_data.py is a handy function for Gym users who want to create 'special' datasets of spectral indices. Currently, only MNDWI and NDWI are supported.

It would be great to add a few more common ones, (see, for example https://awesome-ee-spectral-indices.readthedocs.io/en/latest/list.html#)

Perhaps the easiest way to do this would be to use an existing library e.g. https://github.com/awesome-spectral-indices/awesome-spectral-indices, but , but this would require a fairly major rewrite because of the bespoke way datasets are crated from files, not arrays ....

Perhaps for now, very common general (multi-sensor) indices like BNDVI , GBNDVI, and ExG (for vegetation), NBR (for burn) , and NDVIMNDWI and NDTI (for water) could be incorporated