added clear method; return None for empty boxes

mathentry.py

@@ -25,6 +25,9 @@ class MathEntry(wx.TextCtrl):
     def GetValue(self):
         val = super().GetValue()
 
+        if val == "": #TODO?
+            return None
+
         if val == "None": #TODO?
             return None
 
@@ -90,4 +93,8 @@ class MathEntry(wx.TextCtrl):
         self.SetForegroundColour(wx.NullColour)
 
 
+    def clear(self):
+        super().SetValue("")
+
+