encap_sweeper: Add `spread` and `as_encap_sweeper`.
interfaces: Add `spread` interface and split interpolation.
poly_interp: Split `interpolate` and `interpolate_initial` routines.
mlsdc: Add `set_nsweeps`.
pfasst: Fix predictor.
mpi_pfasst: Set nsweeps on coarse level to 2.
Merge pull request #59 from memmett/feature/fix-pf-interp

Fix PFASST predictor.  Fixes #58.