Vector pp rules for time coordinates

Replacement for @pp-mo's #1239. This builds on #1239 but reinstates the scalar version of the time pp rules. The vector version is not used (yet). I've removed some of the previous unit and tests and added others.
I know you inherited these tests ...

I had to look this up in `iris/docs/iris/src/developers_guide/tests.rst` ... test class names of functions should be of the form `TestEmpty` and `TestSingleArg` etc