Make date/datetime types consistent

[marksteward]

The algorithm generator creates PSEvents with date set to a datetime, while get_manual_ps_dates sets it to a date. Make these consistent and testable. Make sure overriding starts and ends in the json works too.

[jaylett]

Should we also be ensuring we are TZ-aware throughout?

[marksteward]

Yes, but I'd like some more tests before we start that :)

E.g. https://github.com/pubstandards/pubstandards-london/blob/master/tests/test_ps_data.py#L71

[Jonty]

Argh, I wrote "timezones" on my notepad on the train and then forgot about it. Will do that tonight.

[marksteward]

TZ-awareness is #9, which is probably best sorted out before addressing this, as it also should include tests for starts/ends override.

[marksteward]

PSEvent should now consistently have the following attributes:

.date is a datetime.date
.starts/ends are datetime.times
.start_dt/end_dt are aware datetime.datetimes