PFASST: Reduce to MLSDC when only one time-processor is used.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge branch 'feature/new-quadrature' of github.com:Parallel-in-Time/PFASST into feature/new-quadrature

Signed-off-by: Matthew Emmett <memmett@gmail.com>