Adapt several linker algorithms to Linker base class

Rebase of !164 (closed).

From !122 (closed) and @raaij:

The way MC links are handled will likely change in the future. To make things forward compatible and allow conversion of the current links to Transformers while also keeping things working, a new base classes for the linkers are proposed that inherit from (Multi)Transformer. They have a few convenience methods to ease the transition. The 2 VP and an OT associator have been converted to this new base class as a test case.