Moved newH into base class, ConstraintImplementation

- Derived implementations of ConstraintImplementation::doBuild will be
frequently called, so effeciency is key.  SparseVector::reserve
allocates the same amount of space on the heap in every call, so it can
be called in the constructor.
- Most implementations of ConstraintImplementation::doBuild will use a
sparse newH, so the value is moved into the base class and set up in the
constructor.
Rename MlcpPhysicsProblem::updateConstraint

- Added doxygen documentation for function
- Updated syntax for parameters
Add test for MassSpringRepresentationLocalization::isValidRepresentation

- Add template class for creating a MockDescendent
Minor comment changes in mass spring Contact & Localization unit tests
Remove m_newH initialization from ConstraintImplementation

It's clearer for derived classes to reserve memory in m_newH as needed
in doBuild, and because m_newH is a member variable, it does not incur
extra cost to reserve the same amount of space more than once after
resizing.