Expand Functions introductory notebook (#211)

* Add new exercises and a first test

* Fix a few bugs with the tests

* Revert "Fix a few bugs with the tests"

This reverts commit 8985fb9.

* Improve tests parametrization

* Add test to new exercise #2

* Update toc

* Improve test on ex.2

* Add a third exercise

* Simplify test

* Apply suggestions from code review

Co-authored-by: Despina Adamopoulou <[email protected]>

* Revert "Apply suggestions from code review"

This reverts commit 509312a.

* Fix errors with the last code review

* Update yards and feet conversion hint

* added missing commas

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Co-authored-by: Despina Adamopoulou <[email protected]>
Co-authored-by: Despina Adamopoulou <[email protected]>