"target_dimension" in aggregate_spatial

[soxofaan]

@Pratichhya raised question what "target_dimension" in "aggregate_spatial" is for, it doesn't seem to do anything.

background info:

• drop target_dimension in aggregate_spatial? openeo-processes#366
• https://github.com/Open-EO/openeo-processes/pull/382/files#diff-ec59ca657408ec781d5c1f4b0d5a7d8a1f6072a27cc9c2a9bf1d617c1d72a4b1R84

We don't seem to implement it, so I think we better throw an "not implemented" error at the moment when a user tries to use it instead of blindly doing nothing

[Pratichhya]

To give more context on my side, I confused it as a process that will help me rename the band label(https://open-eo.github.io/openeo-python-client/api.html#openeo.rest.datacube.DataCube.aggregate_spatial) that we get as a result of aggregate_spatial.

For example, rename the unnamed_band from this notebook to an ndvi.

[soxofaan]

I just added some FeatureUnsupportedException to at least make it more clear we don't support it at the moment