Feature/cg condition estimate

Support a CG estimate according to Saad. Trivially works in parallel and should be accurate enough for most applications. Performance of CG without condition estimate should not be affected.

Don't know how it could be used for complex numbers, so for now, I just convert the CG coefficients to reals to ensure it at least compiles in that case.

I also fixed the ARPACK wrapper for matrix sizes < 20x20. (Before, it always told ARPACK to construct 20 Arnoldi vectors, which obviously does not work in those cases)

@dominic As for the test, it also passes if no Arpack is found. I guess this is the correct behaviour? (The estimation code also compiles fine without arpack, it only gives a cerr output if Arpack is not present and returns -1 as condition estimate)