License detection

We expect licenses declaration in notebooks to be declared with the specific SPDX identifier of the license in the metadata of the notebook:

{
   ...
   "metadata":{
      "kernelspec":{
      ...
      },
      "language_info":{
      ...
      },
      "license":"MIT",
   }
   ...

}

We believe this is easy enough and offers enough flexibility. A special id "PROPRIETARY" can be used to identify proprietary notebooks.

Please note that you can also simply drop a "License.txt" file in the root of your project to declare a license for all the notebooks contained: in that case we suggest you use standard SPDX templates.