Implemented stimulus generators

- Implemented stimulus generators.
- Added better handling of HEKA digital I/O channels. Digital channels can now be treated in the same way as analog channels. The core now handles all the necessary bit shifting and masking.
- Fixed some issues preventing the use of metric prefixes. You can now specify stimuli in the appropriate units (e.g. "mV" or "pA") and the core will handle it appropriately.
- Started a unit test suite for SymphonyUI.