python: better way to extend existing classes

* wrap/python/spot.py: Use a decorator to extend classes.
* wrap/python/tests/formulas.ipynb: Adjust expected help text.

wrap/python/tests/formulas.ipynb

@@ -243,9 +243,9 @@
        "output_type": "stream",
        "stream": "stdout",
        "text": [
-        "Help on function _formula_format in module spot:\n",
+        "Help on function __format__ in module spot:\n",
         "\n",
-        "_formula_format(self, spec)\n",
+        "__format__(self, spec)\n",
         "    Format the formula according to `spec`.\n",
         "    \n",
         "    Parameters\n",
@@ -724,4 +724,4 @@
    "metadata": {}
   }
  ]
-}
+}