Nodes which are a function of time

As specified in #38. __Depends on #32.__

Nodes marked as a function of time are evaluated in advance and played back on the SpiNNaker board.

Greatest support is for periodic Nodes, though aperiodic Nodes are supported given a bounded simulation run time and sufficient memory.

- [ ] Raise Exceptions if ineligible Nodes are marked
- [ ] Raise better Exceptions in the case of insufficient memory: _ideally it would be nice to dynamically switch from on chip to on host in this case, but it's currently noticed too late in the build process.  Perhaps an intermediate representation between Nengo and PACMAN would be good after all..._