Add ability to do relative AV sync timing measurement

[matt-hammond-001]

Add a mode of operation that can compare the relative timing between one input (light sensor or audio) and one or more other inputs.

This mode of operation does not need to use the DVB CSS protocols at all. Instead it can just take measurments then use one as the "expected" timings to compare against the others (treating them as the "observed" timings).

For two sets of measurements being compared, the error bounds need to be combined, but not just by summing (since some effects are not independent variables - e.g. uncertainty due to sampling being quantised into 1ms period chunks)

[matt-hammond-001]

Work completed and merged.

[matt-hammond-001]

Undoing mistake closing the wrong issue