Address more TODO ISO comments.

(and issues arising from).

@arjclark - please review.
@benfitzpatrick - we can't use this try-block test in the intended way (it should fail if the final cycle point is not an interval) with integer cycling because even though we can now specify integer intervals explicitly (e.g. ```final cycle point = P5```) ```get_point_relative()``` still succeeds with the ambiguous input (```final cycle point = 5```).  Can we drop the latter behaviour now? (i.e. assume/require all integer intervals have a "P")?