[GeoMechanicsApplication] Document the relation between the UPwSmallStrainInterfaceElement and UPwBaseElement

[rfaasse]

As a developer, I'd like to know the relation between the UPwSmallStrainInterfaceElement and UPwBaseElement, such that I know the steps needed to isolate the interface elements from the continuum elements.

Predecessor #12493

Acceptance Criteria

• The functions used in the interface elements, but not overridden (i.e. only available in UPwBaseElement) are documented
• The variable JOINT_WIDTH is understood and documented

[markelov208]

UPwSmallStrainInterfaceElement calls the following UPwBaseElement functions explicitely

1. Check
2. Initialize
3. GetNumberOfDOF
4. CalculateIntegrationCoefficients
   Based on test_interface.py (including "solution_type": "dynamic"), it also uses the following UPsSmallStrainElement functions:
5. GetDofList
6. CalculateLocalSystem
7. CalculateRightHandSide
8. EquationIdVector
9. GetIntegrationMethod
10. GetDofs
11. CalculateDerivativesOnInitialConfiguration
12. CalculateJacobianOnCurrentConfiguration
13. CalculateDampingMatrix
14. GetSecondDerivativesVector
15. GetFirstDerivativesVector
16. GetStressStatePolicy

The following UPwSmallStrainElement functions are not covered by the tests:

1. CalculateLeftHandSide
2. GetValuesVector
3. CalculateOnIntegrationPoints
4. SetValuesOnIntegrationPoints
5. ResetConstitutiveLaw

However, these interface tests are not complete. For example, they do not cover all UPwSmallStrainInterfaceElement functions, namely, the following 6 functions out of 42 functions are not covered:

1. Create(IndexType NewId, GeometryType::Pointer pGeom, PropertiesType::Pointer pProperties)
2. CalculateOnLobattoIntegrationPoints( Variables Vector and Matrix)
3. ExtrapolateGPValues (it can be activated with NODAL_SMOOTHING)