Add support for user timers in iotile-sensorgraph

We currently only support a single global 10 second tick for historical data and a high speed 1 second tick for realtime data. Timers then have to be static since their rules are defined as a hardcoded number of ticks.

In order to support easy configuration of how often data is taken, we want to add a user controllable tick that can be easily set. This needs support in `every` clause constructs and gating in `when` clause constructs.

- [ ] Add support for 2 user ticks
- [ ] Add support for config variable declaration setting their time intervals
- [ ] Add support for triggering `every` clauses based on user ticks
- [ ] Add support for gating user ticks in `when` clauses
- [ ] Add warning messages for using a user tick that does is not initialized with a default interval
- [ ] Add support for user timers in sensorgraph emulator