Reduce overheads and support dithering

[albireox]

This PR focuses on improving overheads during normal tiled observations. It also enables dithering. It is meant to run alongside lvmguider 0.4.0.

• Remove calls to lvmguider stop --now. The new stop command always cancels the guider task.
• Added pre-readout hook for Exposure.
• Allow GortObserver to read out an exposure asynchronously.
• Gort.observe_tile() will by default read the last exposure asynchronously to allow for the next slew and acquisition to happen during readout.
• Added a context manager GortObserver.register_overhead() that measured the elapsed time in different parts of the slew and acquisition process and records the values to the database.
• The dither positions associated with a Tile are now a list of positions that the scheduler wants us to observe. Gort.observe_tile() will try to observe all dither positions without stopping the guider.
• Added Gort.observe() with a simple loop for observing tiles.