Re-enable notebook 14 in run-notebooks gh action

[contsili]

For #504 we temporarily skipped 14_longitudinal_modelling.ipynb in .github/workflows/run-notebooks.yml because we were getting the error:

Could not find pcntoolkit_resources/data with fcon1000.csv and LNM_data.csv

Once this GSoC project is done re-enable notebook 14 in run-notebooks gh action

[contsili]

Also 14_longitudinal_modelling.ipynb is skipped from doc/convert_notebook.py

keep in mind that 14_longitudinal_modelling.ipynb has invalid JSON structure. o this will need to be fixed before running doc/convert_notebook.py