Removed deprecated np.float

numpy 1.20 deprecates np.float, which was only ever an alias for
``float``.  I've replaced all occurrences of np.float with float, as
recommended.  There should be no change in functionality.

openflexure_microscope/json.py

@@ -21,8 +21,8 @@ class JSONEncoder(LabThingsJSONEncoder):
         # Numpy integers
         elif isinstance(o, np.integer):
             return int(o)
-        # Numpy floats
-        elif isinstance(o, np.float):
+        # Numpy floats are just Python floats
+        elif isinstance(o, float):
             return float(o)
         # Numpy arrays
         elif isinstance(o, np.ndarray):