move existing OGS-specific codes in FemLib into OGS to make FemLib as a general FEM lib
remove FemElem.* in MeshLib
add Integration in MathLib
use Eigen for local matrix and vector for this preliminary implementation
add a shape function class for quad 4
add an interface of gauss integration class to use dynamic polymorphism
add a template class of Gauss quadrature for regular shape elements, i.e. line, quad and hex
add an extrapolation class for Quad element with gauss point values
add a struct storing shape function matrices
add a class responsible for mapping between physical and natural coordinates
prepare a template class for isoparameteric finite elements
define QUAD4 finite element
include all the new directories in FemLib
add a test for FemLib::QUAD4