Fixed camera's read_config method

read_config was only returning keys already present in the camera's config dictionary, which meant that if the camera settings were reset/deleted, it no longer read the settings from the picamera object.  I've changed it to use CONFIG_KEYS instead, which I believe is the correct behaviour.

openflexure_microscope/camera/pi.py

@@ -131,7 +131,7 @@ class StreamingCamera(BaseCamera):
         }
 
         # PiCamera parameters (obtained directly from PiCamera object)
-        for key, _ in self.config['picamera_settings'].items():
+        for key, _ in CONFIG_KEYS['picamera_settings'].items():
             try:
                 value = getattr(self.camera, key)
                 conf_dict['picamera_settings'][key] = value