Apply attribute name validation to entire declarative interface

In this commit, the ValidationMixin is applied to all of the objects
in the declarative plotting interface intended for external use.

src/metpy/plots/declarative.py

@@ -531,7 +531,10 @@ def _levenshtein(s1, s2):
             return distances[-1]
 
         whitelist = ['ax',
+                     'data',
+                     'handle',
                      'notify_change',
+                     'panel'
                      ]
 
         whitelist.extend(self.trait_names())
@@ -560,7 +563,7 @@ class Panel(MetPyHasTraits):
 
 
 @exporter.export
-class PanelContainer(MetPyHasTraits):
+class PanelContainer(MetPyHasTraits, ValidationMixin):
     """Collects panels and set complete figure related settings (e.g., size)."""
 
     size = Union([Tuple(Union([Int(), Float()]), Union([Int(), Float()])),
@@ -1238,7 +1241,7 @@ class ColorfillTraits(MetPyHasTraits):
 
 
 @exporter.export
-class ImagePlot(PlotScalar, ColorfillTraits):
+class ImagePlot(PlotScalar, ColorfillTraits, ValidationMixin):
     """Make raster image using `~matplotlib.pyplot.imshow` for satellite or colored image."""
 
     @observe('colormap', 'image_range')
@@ -1293,7 +1296,7 @@ def _build(self):
 
 
 @exporter.export
-class ContourPlot(PlotScalar, ContourTraits):
+class ContourPlot(PlotScalar, ContourTraits, ValidationMixin):
     """Make contour plots by defining specific traits."""
 
     linecolor = Unicode('black')
@@ -1342,7 +1345,7 @@ def _build(self):
 
 
 @exporter.export
-class FilledContourPlot(PlotScalar, ColorfillTraits, ContourTraits):
+class FilledContourPlot(PlotScalar, ColorfillTraits, ContourTraits, ValidationMixin):
     """Make color-filled contours plots by defining appropriate traits."""
 
     @observe('contours', 'colorbar', 'colormap')
@@ -1514,7 +1517,7 @@ def draw(self):
 
 
 @exporter.export
-class BarbPlot(PlotVector):
+class BarbPlot(PlotVector, ValidationMixin):
     """Make plots of wind barbs on a map with traits to refine the look of plotted elements."""
 
     barblength = Float(default_value=7)
@@ -1550,7 +1553,7 @@ def _build(self):
 
 
 @exporter.export
-class PlotObs(MetPyHasTraits):
+class PlotObs(MetPyHasTraits, ValidationMixin):
     """The highest level class related to plotting observed surface and upperair data.
 
     This class collects all common methods no matter whether plotting a upper-level or