Add defocus option to camera Focusers

[AnthonyHorton]

Is your feature request related to a problem? Please describe.
For precision photometry of bright targets it can be advantageous to deliberately defocus the images. While this can be done manually (with the .move_by() method of Focuser classes) it will be overidden by the autofocus that gets triggered at the start of POCS observations.

Describe the solution you'd like
The Focuser classes should have an option to apply a set amount of defocus at the end of the autofocus sequence. The amount of defocus that is applied should be configurable on a per camera basis, and it should be possible to enable/disable defocus for specific targets. Ideally it would be possible to scale the amount of defocus, relative to each camera's default defocus, on a per target basis (brighter target may benefit from more defocus, while fainter ones may want less to reduce the read noise penalty).

Describe alternatives you've considered
'Manual' observing to avoid the triggering of autofocus. This has been done once with Huntsman and the large amount of lost automation makes this far from ideal.

Additional context

[wtgee]

@AnthonyHorton what does "relative to each camera's default defocus" look like? Number of steps or some kind of percentage?

[AnthonyHorton]

@wtgee My thinking is that each camera, in its config, has a default amount of defocus in terms of number of encoder steps, and then targets can apply a multiplier to that default if they want more or less than the default amount of defocus.