(Not recommended) Assemble boundary condition contributions

`assemb`

is not recommended. Use `assembleFEMatrices`

instead.

As explained in Elliptic Equations, the finite element matrices and
vectors correspond to the *reduced linear system* and are the
following.

`Q`

is the integral of the`q`

boundary condition against the basis functions.`G`

is the integral of the`g`

boundary condition against the basis functions.`H`

is the Dirichlet condition matrix representing*hu*=*r*.`R`

is the Dirichlet condition vector for`Hu = R`

.

For more information on the reduced linear system form of the finite element matrices,
see the `assempde`

More About section, and the linear algebra approach detailed in Systems of PDEs.