add Integration/GaussLegendre in MathLib
add a shape function class for quad4
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
add a subdirectory NumLib
add FindEigen.cmake
add CMake options OGS_USE_EIGEN
add Fem Quad4 test with Eigen
cofigure Eigen in travis