make gotoaz()/movement commands in general to be non-blocking

[fergusL]

Anthony advice from slack:

In POCS we use the threading module in the Python Standard Library.

In that module there's Thread, Timer, Event and Lock. Those are the bits you'll need.

https://docs.python.org/3/library/threading.html

For the non-blocking moves the basic idea is that you write a function that contains the while loop that checks when it's time to stop the move, you create a Thread and pass it that function, start the move, start the Thread, then return. The Thread will run in the background and stop the move then at target. (edited)

You can use Events to do simple communication between threads, e.g. you can pass one to the dome movement thread and use it to stop the move early if the abort function is called.