Make allow_none functional in the Date trait type

[mdickinson]

This PR makes it possible to prevent assignment of None to a Date trait. (As usual, it doesn't prevent a default value of None.)

Note that before the recently-merged PR #1429, Date was an instance of BaseInstance, and so in theory should already have allowed and respected allow_none. But a different bug (#495) means that allow_none was not respected. So this PR actually introduces a change in behaviour (effectively a bugfix) for anyone already doing: foo = Date(allow_none=False): before this PR, assignment of None would still have been permitted for such a trait declaration. After this PR, it won't be.

In detail, we:

• Add a new parameter allow_none to Date, defaulting to True for backwards compatibility reasons
• Prevent assignment of None if allow_none is False
• Add some unit tests.

Checklist

• Tests
• [ ] Update API reference (docs/source/traits_api_reference) N/A
• [ ] Update User manual (docs/source/traits_user_manual) N/A
• Update type annotation hints in traits-stubs

[rahulporuri]

LGTM