Doing this in a destructor is the only place where we can guarantee that
no open file descriptors will be left behind.

I was running a load method on some object in a loop, which was
returning false most of the time, and eventually the process run out of
file descriptors because the file was not closed properly.

The original structure was not a set in the mathematical sense, but a
map - renamed to reflect this.

- missing 'thisMultiplicator' in two addVectors calls
- swapped 'x' and 'b' in getResidue call
- added preconditioning
- added bicgstab-exact-residue parameter
- general cleanup