Validate/run doctests when running the tests

I could run the tests with python run_tests.py, though a few failed. Looking at the doctests, some appear to be invalid or incomplete.

Would be nice to fix these, so they can also be used as tests for the code and API.

Here's an example output of running the current doctests:

$ python -m doctest cf/units.py 
**********************************************************************
File "cf/units.py", line 11, in units.Units
Failed example:
    help(cf.Units())
Exception raised:
    Traceback (most recent call last):
      File "/home/kinow/Development/python/anaconda3/lib/python3.7/doctest.py", line 1329, in __run
        compileflags, 1), test.globs)
      File "<doctest units.Units[0]>", line 1, in <module>
        help(cf.Units())
    NameError: name 'cf' is not defined
**********************************************************************
1 items had failures:
   1 of   1 in units.Units
***Test Failed*** 1 failures.