Add fixed scheduler

Unlike the other schedulers this one does not try to expand the graph but leaves
parallelisation decisions to the user. However, the new approach also allows
arbitrary number of outputs which makes broadcasting data straightforward.
Add copy task which copies data from input 0

By itself this task is not very useful but in combination with the fixed
scheduler, broadcasting schemes can be implemented.
Merge pull request #41 from ufo-kit/fixed-schedule

Fix #36: Add fixed scheduler for more user control