ENH: runTimeControl - enable resetting the trigger to an earlier instant

The runTimeControl function object can activate further function objects using triggers. Previously the trigger index could only advance; this change set allows users to set smaller values to enable function object recycling, e.g.

Repeat for N cycles:

1. average the pressure at a point in space
2. when the average stabilises, run for a further 100 iterations
3. set a new patch inlet velocity

• back to (1)

Added a 'none' condition that acts as a pass-through/no-op with the option to set a new trigger value.

Refactored the averageCondition run-time condition and valueAverage FO to use a common base