ENH: Simplify ImageToTubeRigidMetric.

Change-Id: I865b5cc8e0a4904905963de285af7b644f48d1bf
ENH: ImageToTubeRigidMetric: use CompensateSummation.

Improves robustness.

Change-Id: I7100e7328223d34cd4525e875ce6aeea2fb393a6
ENH: Factor out tube point weight calculation.

Change-Id: Ieea9acbd082daa5d1fd3c97a5b8593297ad43ed5
Merge pull request #99 from thewtex/ModelPointWeights

Model point weights