Mesh recombine

[pp-mo]

First-version for "recombining" sub-mesh information.

It's worth noting that supporting location-index-sets would make this easier, as then the sub-region cubes would simply be referred to a common mesh+location, and the awkward 'special index coords' would not be required.

Without that, though, various alternative APIs are feasible + I could still be persuaded to change.
In particular, we could replace 'full_mesh_cube' with 'mesh' + 'location' args -- it's effectively more logical, as we can deduce everything else from the region cubes.
So, that might be slightly neater, but more work is required . Where elements could map both mesh and non-mesh dims, it could get complicated.

TODOs:

• tests for all the consistency checks
• tests for other features + functions
  • transposed forms
  • zero + multiple non-mesh (structured) dimensions
  • dtypes
  • mask behaviours
  • NaNs
• tests for lazy behaviour
• benchmarks showing sensible use of memory in lazy evaluation (as this is a known practical requirement)

[bjlittle]

@pp-mo Looks good... still need to go through the tests, but I can sweep through them later when they're all there 👍

[pp-mo]

A point that just occurred to me..
I have realised that the indices could be checked against the original mesh ~fairly cheaply.
I.E. as everything has coords, we could test that each submesh cube has
all(submesh.coord(xyname).points == meshcube.coord(xyname)[submesh.indices].points)

It would be sort-of logical to do this, to test the user contract that the submesh-s 'belong' to the original mesh.
But I'm thinking we don't have much enthusiasm for this, even though the runtime cost is probably not very high ??

[pp-mo]

Ok, some rather noisy changes there.
I think I covered everything.
Please re-review @bjlittle

[pp-mo]

Update : added tests for various calculation behaviours.
TODO: tests for input error checking (now done)

[pp-mo]

Update: added the rest of the tests.

I think this is now GTG

[bjlittle]

@pp-mo This is a lovely contribution, awesome thanks 🍻