Don't mask another object's name
Don't mask another object's name

This was a nasty trick - importing the new threshold class using the
old class name. This led to much confusion when results from
"DispersionThresholdDebug" did not appear consistent between the
image viewer and dials.estimate_gain while working on #946!

dials/dials,dials/dials,dials/dials,dials/dials,dials/dials