LAMA to Dask: reinstate testing of arbitrary-base `Data.log`

Un-comment to re-add the following test, deactivated in #300, under the test_Data_log unit test, which can be done with the corresponding assertions passing once __itruediv__ is converted ('mask' in the true comment below should read '__itruediv__', that's a copy-and-paste typo):

# Test an arbitrary base, using 4 (not a special managed case like 10)
# TODODASK: reinstate this assertion once mask property is
# daskified.
# a = np.array([[4, 16, 4**3.5], [0, 1, 0.25]])
# b = np.log(a) / np.log(4) # the numpy way, using log rules from school
# c = cf.Data(a, "s")
# d = c.log(base=4)
# self.assertTrue((d.array == b).all())
# self.assertEqual(d.shape, b.shape)