Implement "quantiles" process for the ODC backend

[pierocampa]

The quantiles process is very useful for EO and climate related use-cases, so having it in the catalogue is a major progress.

Quantiles shall be able to ignore nodata, and shall be able to accept n-dimensional arrays as input (if I do understand correctly the specs).

[clausmichele]

Let's define some scenarios:

1. Compute quantiles along single dimension: using apply_dimension() or reduce_dimension()
2. Compute quantiles along multiple dimensions: ? I don't know how to express this as an openeo process

Current implementation is here and ignores by default nodata (for float values):
https://github.com/SARScripts/openeo_odc_driver/blob/f34cd35107e4fb137fc1d23cae246ed362517c75/openeo_odc_driver.py#L824

[pierocampa]

Ok, I see the reduce_spatial process for applying the quantiles process on the XY plane.

For the "any" dimensions case... that might be a questions for the next developers' meeting. !