LAMA to Dask: reinstate string-like data equality test

[sadielbartholomew]

(For the lama-to-dask branch RE the overarching #182 task,) once NCAS-CMS/cfdm#172 is resolved, the following assertion can be reinstated in the test_Data_equals test:

cf-python/cf/test/test_Data.py

Lines 317 to 320 in 629368b

	# Unlike for numeric types, for string-like data as long as the data
	# is the same consider the arrays equal, even if the dtype differs.
	# TODO DASK: this behaviour will be added via cfdm, test fails for now
	# ## self.assertTrue(sa1.equals(sa2))

Presently it is commented out as shown because it is failing on lack of appropriate behaviour in cfdm.