Draft VecMatOnMeshLib (fomer DiscreteLib)

This pull request is intended for having discussion on my draft implementation of VecMatOnMeshLib (previously named DiscreteLib). In the new proposal, there are no linear algebraic stuff such as global matrix, vector and linear solver, which were included in previously proposed DiscreteLib. The linear algebraic stuff have been merged in MathLib/LinAlg. Hence responsibility of the library gets smaller and is focused on the followings:
- allocation of vector and matrix according to mesh partitioning
- association of mesh entity values and linear algebraic data, i.e. vector and matrix
- construct vector and matrix by mesh entity wise operation

To get overview of the library, I recommend you to first take a look at Tests/VecMatOnMeshLib/TestSerialVecMat.cpp

Surely there are many things to be improved in this draft. I welcome any suggestions or comments on its structure, naming and interface.