Add a horrible hack: DuneMatrix class.

This is a hack to get a more efficient constructor for dune-istl matrices from Eigen matrices. It turns out that the existing approach (which is disabled but still in the code) in makeIstlMatrix() is very inefficient. The new constructor cuts time for the full simulator with 18 seconds for SPE9 on my machine.

I realise this is going to be controversial, as it is a deliberate abuse of macro facilities (to make it possible to add a new constructor to an existing class). There are also implementation issues: depending on dune version, some member variables may or may not exist.

So: please discuss this, if it is acceptable, and what alternatives exist if not. But DO NOT MERGE THIS for now.