A plugin to allow recalibration via the web API

This adds a plugin with a single endpoint, "recalibrate".
The recalibrate methods are deliberately simple and not aware
of the StreamingCamera because I want to share the maximum
amount of code with non-microscope projects that also want
to flatten the lens shading.

The plugin is microscope-specific, and therefore handles
shutting down the stream and managing the resolution of
the camera, to allow the calibration to run without
restarting the microscope server.

Currently it **DOES NOT SAVE SETTINGS**.

TODO: save the settings after recalibration.

Arguably this isn't a job for this plugin, what we would
need is:
* A method to sync the StreamingCamera's settings with the
  underlying PiCamera object
* A method to save the StreamingCamera's settings to disk.
I think it would be neatest to provide the first of these as
a method of the StreamingCamera class, and call it from the
plugin.  Saving the settings to disk (e.g. to the microscope
rc file, or to somewhere else) should be a separate endpoint
(although most interfaces will probably call it immediately
after recalibration).

openflexure_microscope/plugins/camera_calibration/plugin.py

@@ -0,0 +1,59 @@
+import time
+import numpy as np
+
+from openflexure_microscope.plugins import MicroscopePlugin
+from openflexure_microscope.utilities import set_properties
+from openflexure_microscope.api.v1.views import MicroscopeViewPlugin
+from openflexure_microscope.api.utilities import JsonPayload
+
+from flask import request, Response, escape, jsonify
+import logging
+
+from .recalibrate_utils import recalibrate_camera
+
+class RecalibrateAPIView(MicroscopeViewPlugin):
+    def post(self):
+        payload = JsonPayload(request)
+        
+        # Figure out the range of z values to use
+
+        print("Starting microscope recalibration...")
+        task = self.microscope.task.start(self.plugin.recalibrate)
+
+        # return a handle on the autofocus task
+        return jsonify(task.state, 202)
+
+class Plugin(MicroscopePlugin):
+    """
+    A set of default plugins
+    """
+
+    api_views = {
+        '/recalibrate': RecalibrateAPIView,
+    }
+
+    def recalibrate(self):
+        """Reset the camera's settings.
+
+        This generates new gains, exposure time, and lens shading
+        table such that the background is as uniform as possible
+        with a gray level of 230.  It takes a little while to run.
+        """
+        scamera = self.microscope.camera
+        with scamera.lock:
+            assert not scamera.state['record_active'], "Can't recalibrate while recording!"
+            streaming = scamera.state['stream_active']
+            if streaming:
+                logging.info("Stopping stream before recalibration")
+                scamera.stop_stream_recording(resolution=(640,480))
+            old_resolution = scamera.camera.resolution
+            try:
+                scamera.camera.resolution=(640,480)
+                recalibrate_camera(scamera.camera)
+            finally:
+                scamera.camera.resolution=old_resolution
+                if streaming:
+                    logging.info("Restarting stream after recalibration")
+                    scamera.start_stream_recording()
+
+