Replace input nodes with state nodes receiving observations

[LegrandNico]

This PR remove input nodes from the API. All nodes are state nodes that can receive observations or be influenced by other nodes, at each time point. Most of the changes are internal, but this will simplify network construction in the future and come with performance improvements.

This also:

• Fix the propagation of prediction errors in case of multivariate ascendency.
• Add better/more relevant examples for multivariate connections.
• Rewrite the updates for the categorical state node (prediction and prediction errors)
• Store the time steps in position -1 of the attributes dictionary
• Allow inputs to receive vector of data, and split predictions and updates in the update sequences
• Remove the method set_update_sequence from Networks
• Add a universal observation step

Close #231 Close #221

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 95.23810% with 15 lines in your changes missing coverage. Please review.

Project coverage is 95.39%. Comparing base (a8d2350) to head (831cd4a).

Files with missing lines	Patch %	Lines
src/pyhgf/model/network.py	89.09%	6 Missing :warning:
src/pyhgf/plots.py	88.88%	6 Missing :warning:
src/pyhgf/utils.py	96.51%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #239 +/- ## ========================================== - Coverage 95.64% 95.39% -0.25% ========================================== Files 23 22 -1 Lines 1194 1130 -64 ========================================== - Hits 1142 1078 -64 Misses 52 52 ```

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