#981: final cycle specification as an offset.

Closes #981 
BTW @benfitzpatrick  - if ```get_point()``` is similar to ```get_interval()```, I noticed while doing #1029 the other day that no exception is raised by the ```get_``` function for a bad input format, you have to call ```standardise()``` to get that.  Is this intentional, and is there some reason not to absorb standardisation into the ```get_``` functions? (I imagine there probably is - just checking...)