add instructions on how to execute the data preparation

[maurerle]

minor fixes for current pandas version

see #21 for details

[jokochems]

Hello @maurerle,

thanks a ton for your effort. I checked it out. Unfortunately, it seems to be not compatible with the older package versions I'm using (numpy 1.22.3 to be more precise). As it is not complete anyways, I suggest to keep this PR open for now until all problems with older package versions are/can be resolved. What do you think? Does this make sense to you?

Again many thanks and best regards,
Johannes

[maurerle]

Of course, sounds good.
Weird that this is not compatible, but no problem then.
This PR is more to prevent the "Yes I had problems too and have been a third through the script but do not remember what has to be changed" situation and document the issues :)

And hopefully it makes things easier when updating it :)

By the way I totally understand the situation of missing time for documentation and keeping repositories updated... :)