Check and revise PA_1_3

• Fixed a few typos and slightly reformulated sentences I found unclear.
• Renamed the variable boolean to temperature_is_nan to make the name more meaningful.
• Ran all code cells (including solutions) to make sure that I do not get an error.
• Adjusted whitespace in a few places to align it with Python conventions.

Remaining issues/questions:

• The link to the book in task 3.1 is not working for me (I get a 404 not found error). I could not figure out what the correct link is.
• Should we give students a hint what function to use in task 1.1? It might take them quite a while to figure out because there are many options (some better than others). Earlier in the week (in In_Class_Activity.ipynb) they used np.loadtxt so they might try that. But it does not handle missing values, so they could get stuck. But then again, if I ask ChatGPT how to read in a scv file with numpy, it directly suggests to genfromtxt and explains that it represents missing data as NaN, so probably a hint is not necessary.
• I was not able to get this Live Share extension to work on my computer (maybe because I am on Linux), so I could not test the tasks in part 3. The instructions seem all clear to me, but maybe someone with a working extension should double-check.