Support clock macro-stepping

Tasks such as I/O and SOCRATES operate at a different cadence to the master model clock. They run at a rate of "ever n ticks" which is implemented by taking the modulo of the step number.

Despite the simplicity of this, there is still room for mistakes but more importantly it's code which science developers should not have to think about.

The requirement is for a mechanism by which a science section can obtain this "every n ticks" from the master clock.

Locate the master model clock and the existing step-number modulo logic used by I/O and SOCRATES. Trace how a science section currently accesses clock state, then define the smallest mechanism for obtaining the every-n-ticks cadence and verify it with the relevant clock or science-section tests.

Written by the indexing model from the issue text.