graham-dds aabedbcac4 Make PoP start and end dates inclusive.
Also removes the clock class.

Makes PoP date ranges inclusive such that a task order with:
-  a start date on or after the current date
and
- an end date on or before the current date
should be considered valid.

This commit also removes the Clock class. This class had two methods as
shortcuts for common uses of pendlum functions. But it wasn't being used
in very many places, and it took up about the same space as

    from pendulum import today()
    ...
    today(tz="UTC").date()

If we want to add this back in, it might be a good idea to extend it for
other time functions we have sprinkled around, like the random date
functions in our tests
2020-01-06 12:29:59 -05:00
..
2019-08-30 09:57:23 -05:00
2019-08-19 10:39:16 -04:00