split-attrs: netcdf saver

[pp-mo]

• changes to netcdf Saver code
  • when saving to nc files, make it respect the local/global status of attributes where possible
    (i.e. typically, if loaded from an original nc file, the local/global status as it was there -- cf split-attrs: netcdf loader #4984 )
• (plus) : a new FUTURE flag implementation to switch between new/old handling behaviour
  • to match existing behaviour AFAP -- cf status-quo tests split-attrs: tests for status-quo #4981

[pp-mo]

In this case, we should get some different behaviour -- at last !
And we will want to expand the tests.

First, I'm expecting to add a new Pytest parameter over "FUTURE.nc_save_splitattrs = (False, True)" to existing tests.
I'm hoping that the differences between old+new results can be captured in the existing testcases (cf #4981)

• duplicate old+new tests with parameterisation
• make testcases switch on parameter, to capture differences between old+new

Also, some specific new behaviour might require additional tests, to run only with "new-style" saving.

[pp-mo]

NOTE: as noted here, I think the special status of attributes called "conventions" (small-c) is probably a mistake, and can be remove.

It was introduced in #711 -- before that, there was already code which would handle "conventions" or "Conventions" specially, using "str.lower()" in several places.
I don't believe lower-case-"conventions" itself was ever a CF term, and I suspect this was probably just defensive programming.
Goes all the way back to the initial commit !

[pp-mo]

#5410 is now the PR for this.