Update when nb metadata is written to notebooks

[mmcky]

trafficstars

In the html pipeline we write a bunch of cell metadata attributes during the execution phase.

It might be better if these are added to the notebook construction phase.

 "metadata": {
  "download_nb": 1,
  "download_nb_path": "https://datascience.quantecon.org/",
  "filename": "overview.rst",
  "filename_with_path": "overview",
  "kernelspec": {
   "display_name": "Python",
   "language": "python3",
   "name": "python3"
  },