changed the numbering of notebooks

renamed all other notebooks to have the correct indexing

slimmmed down all of the material
split the regex part from the pandas notebook into its own seperate notebook

added missing datasets
made some new extra exercises

In python-data-0.ipynb: fixed minor typos.

In python-data-1-recap.ipynb: fixed minor typos and errors, separated
checking code blocks.

In python-data-2-words.ipynb: fixed typos, updated to include
filepaths and changed function arg. names to avoid clash with global
variable "filename".

In python-data-3-numpy.ipynb: fixed typos, and added extra
explanations in exercises at end of workbook on plotting/distance.