interpol uses outdated matplotlib API

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When running through the 'Example: single-null configuration (introduction)' I am getting an error when clicking 'View loaded file' button.

```
Traceback (most recent call last):
File "/Users/timothynunn/INGRID/INGRID/interpol.py", line 235, in PlotLevel
self.psi_levels[label].collections[0].remove()
KeyError: 'psi_1'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/opt/homebrew/Cellar/python@3.10/3.10.17_1/Frameworks/Python.framework/Versions/3.10/lib/python3.10/tkinter/__init__.py", line 1921, in __call__
return self.func(*args)
File "/Users/timothynunn/INGRID/INGRID/gui/ingrid_gui.py", line 516, in ViewData
self.controller.IngridSession.ShowSetup(view_mode=view_mode)
File "/Users/timothynunn/INGRID/INGRID/ingrid.py", line 1323, in ShowSetup
self.PlotPsiNormBounds()
File "/Users/timothynunn/INGRID/INGRID/ingrid.py", line 847, in PlotPsiNormBounds
self.PsiNorm.PlotLevel(self.settings['grid_settings'][k], color=Dic[k], label=k)
File "/Users/timothynunn/INGRID/INGRID/interpol.py", line 241, in PlotLevel
self.psi_levels[label].collections[0].set_label(label)
AttributeError: 'QuadContourSet' object has no attribute 'collections'
```

This is caused by https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.8.0.html#contourset-is-now-a-single-collection

I would suggest the easiest way to fix this is to update the matplotlib requirement in `setup.py`: `matplotlib<3.8.0` and am happy to make this PR.

Start with setup.py and INGIRD/interpol.py, especially PlotLevel, then run the “Example: single-null configuration (introduction)” workflow and click “View loaded file” to reproduce the traceback. Done means the supported matplotlib requirement avoids the removed QuadContourSet API and the example opens without this error.

Written by the indexing model from the issue text.