Add min/max controls to the Jupyter image layer control widgets

Now by showing `my_image_layer.controls` you get widgets that let you adjust
the opacity, stretch function, and data range. The best UI I could come up
with for the data range has "coarse" controls that are typed in manually and
"fine" controls that use a range slider widget.

I note that in my testing, the adjustments don't apply until focus changes to
another widget, which makes for a kind of weak UX. I checked the ipywidgets
bug tracker but don't see any complaints about this; I'm not sure what's going
on, since this seems like a pretty basic piece of functionality.

pywwt/jupyter.py

@@ -118,5 +118,43 @@ class JupyterImageLayer(ImageLayer):
         )
         link((stretch, 'value'), (self, 'stretch'))
 
-        self._controls = widgets.HBox([opacity, stretch])
+        double_width = widgets.Layout(width='600px')
+
+        vrange = widgets.FloatRangeSlider(
+            description = 'Fine min/max:',
+            value = [self.vmin, self.vmax],
+            min = self._data_min,
+            max = self._data_max,
+            readout = True,
+            layout = double_width,
+        )
+
+        # Linkage must be manual since vrange uses a pair of values whereas we
+        # have two separate traitlets.
+        vrange.observe(self._vrange_slider_updated, names=['value'])
+        def update_vrange(change):
+            # Note: when this function is called, these values are indeed updated.
+            vrange.value = (self.vmin, self.vmax)
+        self.observe(update_vrange, names=['vmin', 'vmax'])
+
+        coarse_min = widgets.FloatText(
+            description = 'Coarse min:',
+            value = self._data_min,
+        )
+        link((coarse_min, 'value'), (vrange, 'min'))
+
+        coarse_max = widgets.FloatText(
+            description = 'Coarse max:',
+            value = self._data_max,
+        )
+        link((coarse_max, 'value'), (vrange, 'max'))
+
+        self._controls = widgets.VBox([
+            widgets.HBox([opacity, stretch]),
+            widgets.HBox([coarse_min, coarse_max]),
+            vrange,
+        ])
         return self._controls
+
+    def _vrange_slider_updated(self, change):
+        self.vmin, self.vmax = change['new']

pywwt/layers.py

@@ -601,8 +601,8 @@ class ImageLayer(HasTraits):
         self._stretch_version = 0
 
         data = fits.getdata(self._sanitized_image)
-        self.vmin = np.nanpercentile(data, 0.5)
-        self.vmax = np.nanpercentile(data, 99.5)
+        self._data_min, self.vmin, self.vmax, self._data_max = \
+            np.nanpercentile(data, [0, 0.5, 99.5, 100])
 
         self._initialize_layer()