encap: some templatization tweaks
docu: some docu on IMEXSweeper::predict
some code formatting tweaks

mainly reordered member functions according to order in base classes
exceptions must be thrown to be effective
sweeper: incorporate @memmett's remarks

see https://github.com/Parallel-in-Time/PFASST/pull/77#discussion-diff-16250875
Merge pull request #77 from torbjoernk/feature/sweeper-tweaks

LG