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.