Allow custom chunks

[malmans2]

Is your feature request related to a problem? Please describe.

It's often useful to chunk the requests using various chunk sizes.
#221 fixes chunking with multiple variables, but only size 1 chunks are allowed.

Describe the solution you'd like

I'd like the option to provide custom chunks.
Something like this:

from_source("cds", ..., split_on={"year": 10})

which would result in 10-year chunks.

I.e., split_on should work similarly as the chunks argument of xarray/dask

Describe alternatives you've considered

No response

Additional context

No response

Organisation

B-Open / CADS-EQC