Replace .dat file-ending with similar sensible ending

[fiveop]

The material uses files that end in .dat, spoken “dot dat”. With unclear pronunciation or with bad acoustics this can be misheard as “dot dot” at some point and properly understood at another, which can then lead to confusion: Files are ending in .dot but .gitignore contains *.dat. (Based on recent experience.)

We could replace the file ending .dat with something distinct and recognizable, for example .csv, throughout the material.

[kekoziar]

Changing it to .csv sounds like a good idea. Especially since .dat is often a data file that program developers will encounter, but not researchers (who are more likely to attend these lessons).

Feel free to submit a PR.