Track count of coalescence events

[emilydolson]

Track the count of coalesence events. Requested by @amlalejini.

Should be easy to do by adding a counter that we increment whenever we set mrca to null. Might need to add a couple extra safety checks to make sure we're only incrementing it when mrca has definitely changed, not just when it might have changed. Also need to make sure that lazy mrca evaluation won't introduce inaccuracies (if so, might need to allow turning off lazy mrca evaluation)