list of additional "equalise" operations to assist merge/concatenate

[pp-mo]

Note:
the suggested operations listed in #5375 contain multiple fussy "mismatch" problems like dtype mismatches and minor-floating-point-differences
Inevitably, these probably miss quite a few possiblilties + should be generalised (e.g. across different component types)

[pp-mo]

another similar idea : #5050

This might point to a need to support a generic user-defined equalisation operation ??
It's not clear how that might interact with 'input group selection' ?

[pp-mo]

Initial draft list, drawn (inclusively!) mostly from #5375 :

• equalise coords differing in dtype
• equalise coords with very small floating-point differences (rounding?)
• coords (dim-coords / aux-coords) :
  • equalise or remove where points differ
  • equalise bounds : with/without
• equalise attributes ✔️
• unify time units ✔️
• unify/equalise units (?some specific cases = selected?)
• equalise/remove differing cell-measures/ancils
• equalise/remove cell-methods
• equalise/remove differing derived-coords ((but I think this is a misconception, probably always based on actual coords ??)
• remove/promote selected scalar coords (new-axis, include selected components)
• "merge" attributes (as-per Merging attributes of different cubes before concatenation/merging #5050 : selected ones)
• "custom user function" ?

[pp-mo]

More ideas

• var-names and long-names
  since extending 'LOAD_POLICY' to allow concat, we can expect merge/concat of netcdf data to become more important