Only executed when
smooth.FEM.basis is run with the option
FALSE. It computes the mass matrix. The element (i,j) of this matrix contains the integral over the domain of the product between the ith and kth element
of the Finite Element basis. As common practise in Finite Element Analysis, this quantities are computed iterating over all the mesh triangles.
A square matrix with the integrals of all the basis' functions pairwise products. The dimension of the matrix is equal to the number of the nodes of the mesh.