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.