Simplify BlockVector/matrix #2

[TTitscher]

The Block types implement arithmetic operators without any state of the art expression template lazy evaluation. If we derive the BlockVector directly from Eigen::VectorXd, we only need to provide operators to access a certain .segment for a specific dof type. This may involve some index magic but sounds feasible.

Maybe we can even benefit from the lazy evaluations provided by Eigen::SparseMatrix. But the block operations are limited. Read access is no problem though.

[joergfunger]

I'm not sure if these lazy evaluation optimization really improves the speed, due to our usage of the vectors/matrix. However, I agree that writing our own interface for matrix/vector multiplications is sub optimal. However, I don't see a nice solution in particular for the sparse matrices. Even when we switch to Eigen::SparseMatrix, I would guess that accessing this via block routines is suboptimal (after having checked the documentation, I'm not sure if there are block methods available for sparse matrices anyway)