[Feature Request] Separate dose threshhold for matRad_gammaIndex

[wahln]

@nicverbeek expressed interest to set a separate lower dose threshhold for the gamma Index analysis, e.g. to discard voxels below 10% and also directly enable this setting in matRad_compareDose

Things to consider:

• At the moment, matRad_gammaIndex uses the percentage criterion as lower threshold of the maximum
• Should definition of the threshold be absolute or relative to some value? Relative to maximum dose would be easy
• Both functinos matRad_compareDose & matRad_gammaIndex already have a lot of optional arguments. Can we somehow improve this (use a class, use key/value pairs with the argument parser, etc..)