report volumes in output of overlap_checker

[smason]

have written some code to pull these out of the logs, but having them nicely in the CSV file would make life easier

three columns, vol_hi, vol_lo and vol_shared would be a good trade off I think

might be worth having vol_shared blank for touching objects to indicate there isn't any shared volume. this also provides a natural way to distinguish between objects that have an overlap that rounds (for display) to zero and objects that only touch

[smason]

also allows users to run their own separate checks if they want to do something other than what's done by default