This patch reduces dependencies and makes usage of linear solvers more flexible, i.e. for two problems with different typetag

* Linear solvers don't need type tag anymore, in fact they don't need any template params
* the preconditioner block size is now provided for the solve method (see e.g. staggered newton controller)
* the amg has a convencience alias using a TypeTag but is "independent" through indirection via the traits