TrilinosWrappers::SolverDirect: adjust interface

[peterrum]

... to be able to switch between TrilinosWrappers::PreconditionILU, TrilinosWrappers::PreconditionAMG, TrilinosWrappers::SolverDirect.

The changes are:

• derive from Subscriptor
• allow to pass initialize() an AdditionalData object
• introduce vmult functions
• make SolverControl optional

[tjhei]

Suggested change

	SolverDirect(const AdditionalData &data = AdditionalData());
	explicit SolverDirect(const AdditionalData &data = AdditionalData());

[tjhei]

@p data replaces the data stored in this instance.

[tjhei]

or in the constructor, right?

[bangerth]

Are you deriving from Subscriptor so that you have a base class that you can do a dynamic_cast from? That seems like a questionable thing to do given that the interfaces of the various classes are incompatible. Typically, one would have an interface class where derived classes implement the interface. Would that work?

If that doesn't work, why don't you store these objects via std::variant? This avoids the dynamic_cast from an otherwise rather arbitrary base class.

[peterrum]

@tjhei I have addressed the comments.

Are you deriving from Subscriptor so that you have a base class that you can do a dynamic_cast from? That seems like a questionable thing to do given that the interfaces of the various classes are incompatible. Typically, one would have an interface class where derived classes implement the interface. Would that work?
If that doesn't work, why don't you store these objects via std::variant? This avoids the dynamic_cast from an otherwise rather arbitrary base class.

@bangerth No. The multigrid classes store SmartPointer instances which require that the classes inherit from Subscriptor.

[kronbichler]

Looks good to me apart from a minor typo.