Multithread cpchop

this is a proof of concept of multithreading cpchop.

as dune and opm fight over which framework is the most single-threaded in nature, i have had to do several compromises.

first, i have to do the first subsample in serial. this to get tables and stuff that are built up in a sequential fashion in opm built properly.

then we can multithread, but we cannot multithread the AMG setup phase. thus, this is stuck in a critical section. this will severely inhibit the speedup for problems where this setup time is relevant (typically smaller cases).

finally, you need the p/dominic/umfpack branch from dune-istl (not yet in dune-istl master). superlu is riddled with static variables and is thus utterly useless in multithreaded code.