Matrix powers

PPCG now has the option of using a matrix powers kernel approach which should reduce communication (and hence synchronisation). Due to the nature of the algorithm, this is mutually exclusive with the Block Jacobi preconditioner, but they can be seen as two different approaches to the same problem.

Block Jacobi

New Block Jacobi preconditioner option and a variety of fixes to make problem run faster in general, especially on smaller problem sizes for greater strong scaling efficiency.