OpenAPI Validation Fixes

OpenAPI validation seems to have become stricter about checking for None defaults
in parameters that don't allow None. I have
modified two things as a result:

1. StageTypeProperty (used to pick between SangaStage and SangaDeltaStage)
now defaults to SangaStage (which is the
original behaviour anyway)
2. It is now allowed to pass x,y,z=None when making a move - this is the same as
not passing the respective argument, so the
function already correctly handled this case.

openflexure_microscope/camera/pi.py

@@ -30,7 +30,7 @@ import logging
 import time
 
 # Type hinting
-from typing import BinaryIO, Tuple, Union, Optional
+from typing import BinaryIO, Optional, Tuple, Union
 
 import numpy as np