Merge branch 'karlrupp/feature-pipelined-cg'

karlrupp/feature-pipelined-cg:
Improvements of the conjugate gradient algorithm by using custom kernels
for full use of pipelining and avoiding kernel launch overheads to the extent
possible.
Follows algorithm 2.2 in the paper by Chronopoulos and Gear:
"s-step Iterative Methods for Symmetric Linear Systems",
J. Comp. and Appl. Math 25 (1989).