Add notebook gallery and demo notebooks

[vivienzahs]

A notebook gallery with linked demo notebooks was added to the README.
Therefore, new jupyter notebooks demonstrating the use of different py4dgeo methods in a full analysis workflow were added.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (90506d5) 74.71% compared to head (322fef5) 74.84%.
Report is 2 commits behind head on main.

❗ Current head 322fef5 differs from pull request most recent head 72532f3. Consider uploading reports for the commit 72532f3 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #296      +/-   ##
==========================================
+ Coverage   74.71%   74.84%   +0.13%     
==========================================
  Files          25       26       +1     
  Lines        3247     3252       +5     
==========================================
+ Hits         2426     2434       +8     
+ Misses        821      818       -3     

Flag	Coverage Δ
cxx	87.51% <ø> (+0.42%)	⬆️
python	70.37% <ø> (ø)

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see 2 files with indirect coverage changes

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