Proposed fixes to _discontiguity_in_bounds()

[bjlittle]

@stephenworsley Reference #3404

Let's have the discussion here, and see if there is any momentum.

@pp-mo and @alastair-gemmell

[cpelley]

See #3345

[bjlittle]

Peloton discussion - this passes for meshes that are contiguous, but fails to recognise some meshes that aren't contiguous.

[bjlittle]

@stephenworsley Is this important for iris-esmf-regrid ?

[stephenworsley]

Peloton discussion - this passes for meshes that are contiguous, but fails to recognise some meshes that aren't contiguous.

I've had another look at my comments for this and I believe there is also another bug which would cause discontiguous coords to test as contiguous due to these lines:

iris/lib/iris/coords.py

Lines 1978 to 1984 in ae2b59b

	diffs_along_axis = diffs_between_cells > (
	atol + rtol * cell_size
	)
	points_close_enough = diffs_along_axis <= (
	atol + rtol * cell_size
	)

which would fail when cell_size was sufficiently large.

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