Remove duplicate mesh initialization.

- It is now done in RigidRepresentationBase::doInitialize().
Add isActive() member to Constraint class.

- It returns true if the representations in the Constraint class are
  active.
Consider only active representations for the MLCP problem.
Update the representation mapping after FreeMotion step.

- This is to update the state of the inActive representations.
Merge branch 'fix/stapler-demo-mlcp-explosion' into 'master'

Fix stapler exploding in Stapler demo

+1: Harald Scheirich, Ryan Beasley, Julien Lenoir

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/949143