Merge branch 'manasitiwari/add-pipecg2' into 'master'

Manasitiwari/add pipecg2

See merge request petsc/petsc!3099

KSPPIPECG2: a new pipelined solver that reduces the number of allreduces to one per two iterations and overlaps it with two PCs and SPMVs using non-blocking allreduce.