CSV option to merge feature index into the header?

[m-mohr]

Right now with multiple points in aggregate_spatial and CSV output, I get a csv file with the headers date, feature_index and avg(band_0). It would be nice if I could get something like date, 0_avg(band_0), 1_avg(band_0), 2_avg(band_0) etc. Or do it by default if there's only one statistic.

I think it would also be nice if the dates would be sorted.

[soxofaan]

Current implementation is just a simple ad-hoc solution to have something. Once we iterate further on real vector cube support (e.g. Open-EO/openeo-python-driver#114), issues like this will automatically be covered and improved