MeshCoord cannot compare with an 'equivalent' ordinary AuxCoord

[pp-mo]

The definitive metadata of a MeshCoord is just mesh, location + axis.

These can be leniently compared with another one, e.g. to fetch the matching coord from a cube on a different mesh :
For example...

>>> import  iris.tests.stock.mesh as ism
>>> cube1 = ism.sample_mesh_cube(mesh=ism.sample_mesh(n_nodes=10, n_faces=3, n_edges=0))
>>> cube2 = ism.sample_mesh_cube(mesh=ism.sample_mesh(n_nodes=10, n_faces=4, n_edges=0))
>>> co = cube1.coord('latitude')
>>> co
<MeshCoord: latitude / (unknown)  mesh(<Mesh object at 0x7ff8906fc580>) location(face)  [...]+bounds  shape(3,)>
>>> cube2.coord(co)
<MeshCoord: latitude / (unknown)  mesh(<Mesh object at 0x7ff8906cbdf0>) location(face)  [...]+bounds  shape(4,)>
>>> 

However, a MeshCoord can be "reduced" to an equivalent AuxCoord (i.e. with same names, units, points + bounds).
This happens automatically when a cube is sub-indexed, e.g. "meshcube[..., 2]" or just "meshcube[..., 0:]".
However, the AuxCoord can never "compare with" the original, since the metadata structure is too different.
Hence, code analagous to the above won't work...

>>> import  iris.tests.stock.mesh as ism
>>> cube = ism.sample_mesh_cube()
>>> cube1 = ism.sample_mesh_cube()
>>> cube2 = cube1[..., 0:]
>>> co1 = cube1.coord('latitude')
>>> co1
<MeshCoord: latitude / (unknown)  mesh(<Mesh object at 0x7ff8906590a0>) location(face)  [...]+bounds  shape(3,)>
>>> co2 = cube2.coord('latitude')
>>> co2
<AuxCoord: latitude / (unknown)  [3200, 3201, 3202]+bounds  shape(3,)>
>>> cube2.coord(co1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/h05/itpp/git/iris/iris_main/lib/iris/cube.py", line 1919, in coord
    raise iris.exceptions.CoordinateNotFoundError(emsg)
iris.exceptions.CoordinateNotFoundError: "Expected to find exactly 1 'latitude' coordinate, but found none."
>>> 

I.E. you cannot find the "equivalent" coord in this way, when one is a MeshCoord and the other a 'normal' Coord.
However, that does work for a "normal" cube, including finding matching coords of a different type i.e. DimCoord <-> AuxCoord.

This is likely to cause problems in some generic operations, e.g. regridding.

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[trexfeathers]

See #4751 and #5991