Cache the current frame for fast access

When working with large data on a slow filesystem, simply reading the
current displayed image gets to be really slow. This happens when
checking the intensity of different coordinates for instance (via
mouseover events). To improve performance, we can store the currently
displayed frame and simply access it if the user has not selected a
different frame.

We accomplish this here by storing the frame number (time) and the
current frame. These are both initialized to be `None` so that they will
fail any checks to see if the frame can be reused on the first call.
Within `get_image`, checks are added to see if the requested (or
default) frame number (time) are the same. If not, the new frame is
loaded and the time updated. By only updating the time after, this
ensures that some raised exception doesn't create an incorrectly
determined cached image. In either case the frame just now or already
loaded, is returned.

mplview/core.py

@@ -63,6 +63,8 @@ def set_images(self,
             )
 
         self.neuron_images = new_neuron_images
+        self._cur_img = None
+        self._cur_img_time = None
         self.cmap = cmap
         self.vmin = vmin
         self.vmax = vmax
@@ -123,16 +125,18 @@ def get_image(self, i=None):
         else:
             i = 0
 
-        cur_img = self.neuron_images
-        if (len(self.neuron_images.shape) == 3):
-            cur_img = cur_img[i]
-        else:
-            cur_img = cur_img[...]
+        if i != self._cur_img_time:
+            if (len(self.neuron_images.shape) == 3):
+                self._cur_img = self.neuron_images[i]
+                self._cur_img_time = i
+            else:
+                self._cur_img = self.neuron_images[...]
+                self._cur_img_time = 0
 
-        cur_img = numpy.asarray(cur_img[...])
-        cur_img = cur_img.astype(float)
+            self._cur_img = numpy.asarray(self._cur_img[...])
+            self._cur_img = self._cur_img.astype(float)
 
-        return(cur_img)
+        return(self._cur_img)
 
 
     def color_range_update(self, vmin, vmax):