Add UseEpsilonValuesUnderAnalysisThreshold to Plastimatch gamma

[cpinter]

A problem with using analytics threshold for gamma is that the values below that are set to zero, which, when showing it in Slicer in such a way that the background is transparent makes the values under the threshold also transparent. A possible solution is to use Epsilon (=very small value) insead of 0 for values under analysis threshold.

This could be implemented in Plastimatch gamma as a flag in the algorithm called UseEpsilonValuesUnderAnalysisThreshold or similar.

[cpinter]

Kevin Alexander's clarification:
"When performing the gamma using a threshold dose value (i.e. 40% of maximum dose in the plandose), the resulting gamma distribution has black holes where gamma wasn't calculated. It would be nice to have it set to 0 (i.e. green) even though the gamma technically wasn't computed in those voxels (where the dose is less than the threshold). Make sense? I can generate some screengrabs tomorrow if that would help."

[cpinter]

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/712-add-useepsilonvaluesunderanalysisthreshold-to-plastimatch-gamma/details