Bicgstabl

Adds the BiCGSTAB(L) algorithm for solving linear systems.

Although often IDR(s) is more efficient, for strongly non-symmetric system with large imaginary eigenvalues, BiCGSTAB(L) typically converges faster.

This implementation sometimes does not pass the test and fails with a norm of 1e-15. Advice on how to improve the numerical stability are welcome. To some extend it seems to be a limit to this specific algorithm.