Clock to support step fractions

Science developers often want to know how long a fraction of the step is. Usually a half. This is something the clock should be able to tell them so they don't have to muck about.

They also often want the answer at a certain precision (real kind) so an option to do this rather than returning `r_second` should be considered.

Start by locating the clock implementation and its existing r_second return path; review how step fractions and real-kind precision are represented. Done means the clock can report a requested fraction of a step, including a half step, at the requested precision rather than always returning r_second.

Written by the indexing model from the issue text.