Update PointEstimateMixin to match names of 'zmean', 'zmode', 'zmedian' and not to override existing values unless explicitly requested.

[eacharles]

See discussion here:

https://lsstc.slack.com/archives/CQGKM0WKD/p1717447364186889

Basically, I'd like to propose that we:

"match up the names and put a check in PointEstimationMixin so it doesn’t recompute things are are already in the ancil table."

[eacharles]

closed with #112