- remove most of the metaprogramming kung fu in MathFunctions.h (only keep functions that differs from the std)
- remove the overloads for array expression that were in the std namespace
added function for array.min(), array.max(), matrix.cwiseMin(), matrix.cwiseMax().
The matrix.cwiseMin/Max functions required the definition of the ConstantReturnType typedef.
However, it wasn't defined until after MatrixCwiseBinaryOps was included in Eigen/src/SparseCore/SparseMatrixBase.h,
so I moved those includes after the definition of the typedefs.
tests for both the regular and scalar min/max functions were added as well
