ENH: Add CardiacAgastonMeasures extension

Description:
This module will auto-segment the coronary calcium in Cardiac CT scans
and then calculate the Agatston score and label statistics

Contributors:
Jessica Forbes (UIowa), Hans Johnson (UIowa)
Merge pull request #706 from jessicaforbes/add-CardiacAgastonMeasures-master

ENH: Add CardiacAgastonMeasures extension