Merge branch 'karlrupp/feature-pipelined-bicgstab'

karlrupp/feature-pipelined-bicgstab:
Implements a pipelined version of BiCGStab with substantially improved
performance especially for smaller matrices. The algorithm is a slightly
rearranged version of BiCGStab as given in the book by Y. Saad
and only requires a single host<->device transfer in each step.
A fairly similar algorithm has been proposed in the paper
Jacques et al, "Electromagnetic scattering with the boundary integral
method on MIMD systems", LNCS 1593, Springer, 1999.