[GeoMechanicsApplication] Implement drainage policy

[rfaasse]

As a developer, I want the drainage functionality to be separated from the elements, such that the elements become more maintainable and extendible and easier to understand.

Acceptance criteria

• All functionality related to drainage, which could have different implementations (i.e. transient, drained and undrained) is moved to a separate class using the strategy design pattern see e.g. wikipedia for a bit more info.
• Three strategies/policies are created, all deriving from an abstract interface, to provide the drainage functionality.

As discussed as apart of #12067 the interface for this policy will provide the following two functions to add the coupling, permeability and coupling contributions to the left and right hand side:

• AddToLHS(Matrix& rLeftHandSideMatrix, …, const callBack) and
• AddtoRHS(Vector& rRightHandSideVector, …, const callBack).

where the callBack will hold references on required functions and it will be populated by an element (this implementation detail might still change).

[rfaasse]

@markelov208 @WPK4FEM @avdg81 I added some details to this issue, feel free to add to/correct it.

[rfaasse]

To be split up by @avdg81