Intro questions

Hi @haesleinhuepf,

I have been looking through the introductory part and have a few questions. I don't want to create a separate issue for each of them or make a PR for each before discussing, so I group them here. If you prefer separate issues let me know. And sorry in advance if some of the points are already explained somewhere else and I just missed them:

- in the [basic types notebooks](https://github.com/haesleinhuepf/BioImageAnalysisNotebooks/blob/main/docs/02_python_basics/04_Basic_types.ipynb) you show how to combined strings and numbers. Should 1-2 examples be added with f-strings? I find them extremely useful and now favour them even with beginners over the classic ```my text + str(a)``` approach.
- the [masking numpy arrays](https://github.com/haesleinhuepf/BioImageAnalysisNotebooks/blob/main/docs/02_python_basics/05c_masking.ipynb) seem oddly placed as Numpy hasn't been introduced yet at that point. Maybe it should be moved?
- there's never an introduction to packages. The first time it's used is when the ```math``` module is used, but without much comment. Should there be a short notebook introducing this, and in particular the different variants of import like ```from xx import yy```, ```import xx``` etc.
- in the [custom functions](https://github.com/haesleinhuepf/BioImageAnalysisNotebooks/blob/main/docs/02_python_basics/09_custom_functions.ipynb) notebook, you explain how to document functions but don't show how to document inputs/outputs (e.g. numpy-docs). I think that's actually quite useful. Similarly in that notebook, there's no explanation about how functions can take default parameter values and then be called in different ways e.g. ```myfun(3,4)```, ```myfun(a=3, b=4)``` etc. I often see people confused about this, so I think it would be worthwhile to add.
- in the introduction to image processing, there's never a real intro to Numpy and things are added bits by bits and people referred to your other course ```Bio-image_Analysis_with_Python```. Is that intentional? I just fear that people might be a bit confused and think it would be nice to have one short notebook just about Numpy (without all the indexing, cropping etc. which you nicely introduced then using images). Just:
- what is an array
- why is it useful (simple operations on *all pixels* in one line of code without for loops)
- how to create basic arrays (moving the intro to ```np.zeros``` etc. that is currently in the intro
- what other things one can do with numpy (functions, random generators etc.)

Ok, I think that's long enough... Let me know what you think and I'll make PRs if you agree with any of those!

Cheers,
Guillaume