Add example and test for Kriging interpolation

[cchwala]

As stated here the kriging code is not covered in tests, even though it might look like it is tested based on the name of the test... 🙈

In this PR I will

• add a test for Kriging
• add an example notebook for spatial interpolation where Kriging is used
• unpin scipy and pandas (this is included here because I branched from removed pinning of scipy and pandas #131 which is still open, but can be closed in favor of this PR here)

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (700c768) 73.25% compared to head (708771f) 74.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   73.25%   74.53%   +1.27%     
==========================================
  Files          29       29              
  Lines        1077     1084       +7     
==========================================
+ Hits          789      808      +19     
+ Misses        288      276      -12     

Flag	Coverage Δ
unittests	74.53% <100.00%> (+1.27%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
pycomlink/spatial/interpolator.py	91.11% <100.00%> (+15.20%)	⬆️
setup.py	0.00% <ø> (ø)

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