Visualize: visualization of estimated observable mapping

[Doresic]

I implemented a visualization routine to visualize all estimated observable mappings: linear scaling+offset for relative data and splines for semiquantitative data. The visualization of estimated splines already existed, so I made the linear one and combined them into a single routine. However, I made it so that each can also be called separately.

To make the implementation easier, I had to expand some classes connected to the relative data: inner problem, inner parameter.

During the implementation, a few bugs popped up. I fixed those here as well. I'll comment on the PR so it's easier to distinguish.

The notebooks have also been updated accordingly.

[Doresic]

Boehm Example with one semiquantitative and 2 relative observables

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 21.65605% with 123 lines in your changes missing coverage. Please review.

Project coverage is 83.32%. Comparing base (1a3123d) to head (07913e0).

Files	Patch %	Lines
pypesto/visualize/observable_mapping.py	4.00%	120 Missing :warning:
pypesto/hierarchical/relative/problem.py	80.00%	2 Missing :warning:
pypesto/hierarchical/relative/parameter.py	90.90%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1409 +/- ## =========================================== - Coverage 83.85% 83.32% -0.54% =========================================== Files 160 161 +1 Lines 13107 13226 +119 =========================================== + Hits 10991 11020 +29 - Misses 2116 2206 +90 ```

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