Fix unlimited dimensions in Iris conversion

[pp-mo]

We seem to have a problem with ncdata.dataset_like if we try to save with unlimited dimensions.
Example test code (fails) :

ncdata/tests/unit/iris/test_from_iris.py

Lines 111 to 119 in 9f8de90

	@pytest.mark.skip("'from_iris' not yet working with unlimited dimensions.")
	def test_kwargs_unlimited():
	"""Check iris-save kwargs control : make the time dimension unlimited."""
	cube = sample_cube()
	kwargs = dict(unlimited_dimensions=["time"])
	ncdata = from_iris(cube, **kwargs)
	assert ncdata.dimensions["time"].unlimited is True

Not sure why, but this can wait until after v0.1 to fix.

Note on Relevance

It doesn't really matter that unlimited dims aren't set in the ncdata -- they have no meaning outside of a file, so they can just be adjusted before saving.