#9 Improve comment of class GodOfDof (especially underline the relationship with GeometricMeshRegion).

#820 Model: add a new Newton loop, even if it's rigged at the moment with a value that triggers exit after one iteration.

#951 ShellMatrix: now the operation is set internally by the constructor; the context is expected to provide a method named ShellMatrixOperation().

#820 Poromechanics: fluid mass is currently not currently updated after implicit fluid step, leading to an erroneous residual in solid solve. I introduced a new class which purpose is to hold the cross varf variables, as implicit varf has to update an attribute which was formerly held by Model itself.

#820 Solid solve: add a missing coefficient in the Newton solid solve (1. - porosity for mass operator).

#950 In VariationalFormulation, extend the interface of ApplyBoundaryCondition to enable both pseudo-elimination and penalization.

#952 Fix singleton destruction: use now default destructor and foresee the eventual destruction of the object with atexit. Also use a raw pointer rather than a unique_ptr one, which was overkill in this very specific case.

#949 LocalParameterOperator: make the constructor work with the new interface on the Crtp used to store unknowns.

#945 FSI and Poromechanics compiles as well (even if the latter doesn't run correctly: solid varf has been hastily added and gives away stupid results).

#945 [SAME] Introduce a CRTP to avoid repeating in each VariationalFormulation that needs it the declaration of hyperelastic law and its accessors.