BeaconClock: Add some comments

Signed-off-by: Scott Wilson <i@scottwilson.ca>
BeaconClock: Calc new beats using difference in logical time from beacon time rather than Main.elapsedTime

Signed-off-by: Scott Wilson <i@scottwilson.ca>
BeaconClock: Add global tempo change method

Signed-off-by: Scott Wilson <i@scottwilson.ca>