Clock to support sub-stepping

Some algorithms need to iterate within a timestep. This is currently handled with a simple loop within that algorithm. Unfortunately this leads to a lot of plumbing in the algorithm to support it. e.g. tracking the length of a substep, i.e. `dt_substep`.

A proper solution is needed.

Start by examining the clock implementation and the algorithms that currently use simple loops for sub-stepping. Identify how timestep length and substep state are currently passed through the algorithm, then define what a clock-level solution would need to provide. Done should mean algorithms can iterate within a timestep without extensive substep plumbing.

Written by the indexing model from the issue text.