Timer based meridian flip in the Sequencer

[wannabecoffeenerd]

I have an OnStep based mount which doesn't track beyond the meridian. As a result, the current check in Sequencer.js for hour angle > 12 never gets triggered for the meridian flip since my mount stops tracking usually ~1 min before transit.

As a solution to this, I implemented a timer based solution to wait for the target to cross the meridian rather than the mount crossing the meridian. This way, the mount can stop tracking before the meridian and then issue a precise goto after the target crosses the meridian.

TODO:

• [x] Fix unresponsive slew command in the timer callback
• [x] Properly cancel the timer and reset state after meridian flip
• [ ] Ideally shut down mount tracking when waiting for transit and restart tracking in the timer callback