Add section to modularize previous work to py-dev

I would like to add a section called "Applying what we have learnt {#py-dev-applying-what-we-have-learnt}" or similar to the py-development chapter where the studens use functions to modularize previously written code, but I don't yet know what they will have done at that part. This issue serves as a reminder to revisit once the previous chapters are solidified.
