Improving some small things about self.depth

[johndoknjas]

Inspired by issue #113, it's probably best to change a few things about self.depth. Not required per se, but maybe better practice.

• Have it be an int instead of a string, which is more intuitive. The few places where self.depth is used in the Stockfish class will need to be updated, as well as any tests referencing self.depth.

• Signify self.depth as "private" (so self._depth), and write a public getter function for the user to access it.