[GeoMechanicsApplication] Make UPwBaseElement a non-template class

[markelov208]

📝 Description The step to make non-templated SmallStrainUPwDiffOrderElement class inherited from UPwBaseElement class.

• Removed the template parameters TDim and TNumNodes from the member function signatures.
• Use of TDim is replaced withthis->GetGeometry().WorkingSpaceDimension().
• Use of TNumNodes is replaced with this->GetGeometry().PointsNumber().
• Removed the explicit class instantiations from the implementation file.
• Removed GeoElementUtilities::GetNodalVariableMatrix because it is not used any longer.
• Renamed undrained_terms into transposed_coupling_matrix, that was missed in https://github.com/orgs/Deltares/projects/55/views/1?pane=issue&itemId=66055494
• Fixed a number of code smells. Only one code smell left is 'Remove this call from a constructor/destructor to the overridable "GetIntegrationMethod" method.'