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.
You'd hope, but it's not guaranteed. Testing/Experimentation showed that if `lbft` is a scalar, but `t1` (or `t2`) are vectors - e.g. fixed forecast ref time and a series of validity times, then the logic in the rules function leads to a mismatch of shapes. This could be handled there (but this gets messy), or may be handled in the logic higher up (in `FieldCollation` code) but this is still changing. Given ```broadcast_arrays``` works with views, this seemed a robust approach. I changed the docstring to reflect this change.