Create a preconditioner interface class.

[pelesh]

I think it would be a good idea to create abstract interface for iterative solver preconditioners and expose them to users. Current preconditioners we use should inherit from that interface, as well. I suggest something like this:

• The preconditioner class implements method(s) applyPreconditioner(), possibly overloaded.
• Preconditioner interface is exposed to users so they can write their own preconditioners.
• Iterative solver base class should implement method setPreconditioner, which should basically set a pointer to external preconditioner instance.
• We could wrap preconditioners we currently use in this abstract preconditioner class.

CC @kswirydo