Dim slicing

[pp-mo]

Implements #68
WIP

Initial sketches only so far.
Re-organises the #64 code a bit, and esatblishes 'ncdata.utils' a bit futher.
Includes an ncdata "copy" operation, which we have not previously implemented : That could be treated separately.

STATUS: 2024-04-12

• awaiting proper testing of the dataset comparison operations Compare tests #70
• when that is done, we should properly implement, document + test the proposed copy operation, as seen in code in this PR
  -- (see in copy support #78) which this code absolutely needs, to avoid modifying its input
• only then can we seriously come back here + make this a thing !