Documentation update addressing why not all features have segmented areas

[JuliaKukulies]

This PR resolves #52 which has been a long existing issue reporting that not all feature IDs can be found in the segmentation mask output and that some features result in ncells=0 in the dataframe output from the segmentation (see also #25 ). After discussing this in #52 with @freemansw1 and @zxdawn, we concluded that this is not a bug, but that there are simply some situations where the segmentation cannot create a segmented area even though everything is working as expected.

I have tested and verified this and updated the documentation page to visualize cases identified by @freemansw1. Note also that I have added a short section describing the segmentation output.

Here you can have a look at the documentation page looks like: https://tobac--195.org.readthedocs.build/en/195/

We can also wait until v1.5 but given that this is only documentation and that several users may encounter this issue, we may as well squeeze it into v1.4?

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[codecov[bot]]

Codecov Report

Base: 36.20% // Head: 36.22% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (934c3ef) compared to base (29e5598). Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.4.0     #195      +/-   ##
=============================================
+ Coverage      36.20%   36.22%   +0.02%     
=============================================
  Files             10       10              
  Lines           2116     2120       +4     
=============================================
+ Hits             766      768       +2     
- Misses          1350     1352       +2     

Flag	Coverage Δ
unittests	36.22% <ø> (+0.02%)	:arrow_up:

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Impacted Files	Coverage Δ
tobac/__init__.py	92.00% <0.00%> (-8.00%)	:arrow_down:

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[JuliaKukulies]

Thanks for your reviews @snilsn and @freemansw1! I made some minor changes according to your points and I am ready to merge.

[freemansw1]

I'm happy with your changes and happy for you to merge once @snilsn has a chance to look things over.