refactor(dis*): flatten idomain and move to base type

[wpbonelli]

Store idomain as a 1D array in DisBaseType, rather than the proper grid shape in the concrete types. This deduplicates the variable and simplifes grid equivalence checks added in #2149 and planned in #2153.

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Checklist of items for pull request

• [x] Replaced section above with description of pull request
• [x] Formatted new and modified Fortran source files with fprettify
• [x] Added doxygen comments to new and modified procedures
• [x] Removed checklist items not relevant to this pull request

[mjr-deltares]

A quick note: this will have consequences for anyone using IDOMAIN in its current shapes through the API. Personally, I have code that relies on the current (multi-dim) arrays, for example, to render VTK files. This will break then. I am not trying to block this, because I can easily adapt, just wanted to share the heads up.

[wpbonelli]

After chatting with @mjr-deltares we thought this should wait, at least until a non-bugfix release, since it will break API usages. Maybe also consider alternatives.

@langevin-usgs

[wpbonelli]

closing as low priority, maybe reopen if revisited someday