Automatic publication to prov store and update of README files in Python

Following #86 (and discussions at #84), for each example document (in `examples/spm` and `examples/fsl`) only the `provn` serialisation is stored in the `nidm` repository. A `README` is provided for each `provn` file providing a link to a [public document in the Prov Store](https://provenance.ecs.soton.ac.uk/store/public/), where alternative serialisations (ttl, xml, json, svg, pdf, png) can be found.

This is an extension of #90 to provide a Python version of the script used to automatically:
1. upload the modified documents to the Prov Store and 
2. update the REAMEs accordingly.

This Python version takes advantage of the `rdflib` library and is therefore able to ignore BNode when comparing graphs (which was not available in the bash shell version leading to the creation of duplicates on the ProvStore).