Add "SearchDepth" UCI option

[thearst3rd]

Leaving this here for now. It works, though I'm not totally happy with it.

This pull request adds a UCI option SearchDepth which controls the depth at which the engine will search. This allows you to set the depth of the engine using the menu of a UCI compatible chess engine. I tested it with Cute Chess and it works as intended. Also, the command line option --depth still works as normal.

Here is what I don't like:

I converted the variable depth into a global variable so it persists over time and can be modified when a setoption command comes in. What I wanted to do is remove depth as a parameter from the command function, and just have it read that global variable instead. But python isn't my strongest language, and I can't quite figure out how to get the unit testing suite to be able to set or read that variable. I tried importing it, I tried just declaring it as a global and seeing if it would match it with the global from the other file, but I couldn't quite figure it out. So, maybe you know what I've done wrong here.

So instead, I just kept depth as a parameter, and made a new search_depth global variable. setoption modifies that global variable rather than the local one.

I wanted to write a test case that checked that the setoption command works as intended, but again, I couldn't figure out how to get the testing file to properly read the value! I wrote the following test:

    def test_setoption_searchdepth(self):
        """
        Test setoption command setting the current search depth
        """
        board = chess.Board()
        global search_depth
        search_depth = 3
        command(search_depth, board, "setoption name SearchDepth value 5")
        self.assertEqual(search_depth, 5)

I believe the above test should work, but it says that the assertion failed because 3 != 5. I checked running the command manually in UCI mode, and that command definitely is working as intended. Maybe you know why this test isn't working :P

I don't think this PR should be merged as-is. If you can help me understand python better and get that global variable to be readable/writeable from both files, then we can remove the redundant parameter from command and just have it read the global while also adding in the new test.

Let me know your thoughts! :)