[GeoMechanicsApplication] Extract a static utility function for the calculation of the Stiffness Matrix (K)

[markelov208]

As a developer, I would like to have a utility function which can calculate the stiffness matrix, such that this can be re-used.

Acceptance Criteria

• The calculation of the stiffness matrix is available as a utility function, which takes the B-matrix, constitutive matrix and the integration coefficient
• This calculation is defined once and re-used at all locations where this calculation is done in the geomechanics application.
• The utility function is documented and unit-tested.

Background
This issue handles the K in the equation describing our UPw system of equations:

$$ \begin{bmatrix} M & 0 \\ 0 & 0 \end{bmatrix} \begin{bmatrix} \ddot{u} \\ \ddot{p} \end{bmatrix} + \begin{bmatrix} D & 0 \\ Q^T & C \end{bmatrix} \begin{bmatrix} \dot{u} \\ \dot{p} \end{bmatrix} + \begin{bmatrix} K & -Q \\ 0 & H \end{bmatrix} \begin{bmatrix} u \\ p \end{bmatrix} = \begin{bmatrix} f_u \\ f_p \end{bmatrix} $$

The mathematical definition is:
$$K = \int_\Omega B^T C_{constitutive} B d\Omega$$

Where $\Omega$ is the domain, $B$ is the B-matrix and $C_{constitutive}$ is the constitutive matrix.