Allow element-wise comparisons with None

[davidhassell]

Fixes #281

[davidhassell]

Note to selves: think about this as part of the dask work.

[sadielbartholomew]

All good, assuming the desired behaviour for the specific case Bryan raised (as I am guessing from the thread on the corresponding Issue), along with the obvious general behaviour, is that:

In any case, I'm glad you're putting in a PR so the right error is raised rather than a stackdump.

namely that we return the same error as numpy (#281 (comment)), which is indeed implemented:

>>> d = cf.Data(np.array([1, 2, 3]))
>>> if d != None:
...     print("nope")
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sadie/cf-python/cf/data/data.py", line 1544, in __bool__
    raise ValueError(
ValueError: The truth value of Data with more than one element is ambiguous. Use d.any() or d.all()

[sadielbartholomew]

The linting workflow is failing on some spurious whitespace, which I will deal with after so we can merge this as-is. There is a 3.6 workflow running, too, when it shouldn't as we dropped 3.6, hence the failure of it, so I'll deal with that.