Taking the Dirichlet boundary conditions into account
As the boundary conditions of first type are partialy taken into account during
the computation of the stiffness matrix and right-hand side without boundary conditions and the computation of the boundary conditions of
third type, there are now only two more things to do : to set the values
on the diagonal of the stiffness matrix to "1" and the entries of the right-hand
size corresponding to Dirichlet nodes to the value of the boundary conditions.
For each Dirichlet node, which has i as global number.
Kii = 1
fi = g1(xi);
g1(xi) is the value of the Dirichlet boundary
condition at the point xi.