#981: final cycle specification as an offset.

Closes #981 
It was intentional, just because the `standardise` call is really slow - it
actually parses and re-dumps the date string, instead of just bunging
it as the `self.value` (which is all `get_point` does).

We could probably support `standardise` as a keyword argument to
get_point and get_interval - as long as it can be switched off, it
could even be the default behaviour.