Ensure all VPChannelID values used in associators are PixelID bits only

Fix an issue where the VPChannelID values created by VPDigitLinker were using VPChannelIDs with non-zero values in the "high" bits which are either fractions of a channel or unused. This breaks the later VPFullCluster2MCParticleLinker in Moore. This MR forces all VP associators to use the bits required to set the pixels only so the internal integer is consistent at every step. This was reported in Boole#36 (closed).