smallExample was changed to include two of the examples used in testConstrainedLinearFactorGraph
ConditionalGaussian was changed to make solve() virtual, as this is necessary for ConstrainedConditionalGaussian
''Limitations: ''
* Any given node can only have one constraint on it, but constraints can be of arbitrary size
* Constraints can only be specified as a blockwise system, where each block must be square and invertible to support arbitrary elimination orderings.
* ConstrainedNonlinearFactorGraph is disabled until a better solution for handling constraints in the nonlinear case is determined.