add equality constraint representation

[yetongumich]

Add EqualityConstriant in 3 ways:

• DoubleExpressionEquality: with a scalar expression g(x), forcing g(x)=0
• VectorExpressionEquality: with a multi-dimensional Vector expression g(x), forcing g(x)=0 in all dimensions
• NoiseFactorEquality: forcing a noise model factor to have 0 error, e.g. h(x)-z = 0

This is the first PR. The second PR will contain the penalty method optimizer and the augmented Lagrangian optimizer.

[yetongumich]

Looks really good. I think using my comments it can be even tighter. Consider removing the example.H file and just putting examples inside the test file.

I will also use the example in some future test files, e.g. testAugmentedLagrangianOptimizer.cpp

[yetongumich]

There are a few comments that I cannot resolve, and they need further discussion. I left my reply comments for each of them.

[yetongumich]

Please remove EqualityConstraints. It adds useless code. Unless you can clearly explain its value a normal vector should do.

Removed.

[yetongumich]

Merging, with 2 improvements in the future:

• use overloaded operator in constrained examples
• use template specialization for equalitycosntraints