Restructure the visibility of the descriptors functions

Move some of the descriptor functions: get_rankof_..., get_dataof, ... to descripor_helpers to make them public and prepare the field for new data distributions.

changed milestone to %Chameleon 1.2.0

added API label

requested review from @rperesso

assigned to @faverge

@rperesso @AntJego @mverite (@agullo, @fpruvost) This is a setup to introduce the new data distributions. Please review and approve if you are ok.

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approved this merge request

merged

mentioned in commit 49346b23