Excessive logging information from cf.aggregate with verbose>1

In v3.10.0, setting the cf.aggregate verbose keyword to a value > 1 causes logging output to be returned from internal commands that is not relevant to the aggregation process - namely the cf.CellMethods.equivalent method.

This can be fixed by hard-wiring verbose=1 in these cf.CellMethods.equivalent calls.

PR to follow.

There is also a problem relating to an "environmental` leak", in that setting verbose changes the session log level. Not sure what to do about this just yet.

Are we passing the correct argument to _reset_log_emergence_level?