enhance relational database example

[xrotwang]

The example at https://github.com/fmatter/pyradigms/blob/master/scripts/paradigm_to_relational.ipynb could be fleshed out by

• adding a Language_ID column to the long format, thereby turning the data into a CLDF dataset (modulo some column renaming)
• using pycldf.db.Database to load the data into SQLite

[xrotwang]

@fmatter Let me know if you'd like a PR for the above.

[fmatter]

Absolutely, that would be great! To make version control easier, I've paired the .ipynb file with a .py file using jupytext.