[DOC] Add signal processing tutorial

[Moritz-Alexander-Kern]

This pull request introduces a new tutorial to the documentation, providing users with an introduction to the signal_processing module.

The tutorial is analyzing neo.AnalogSignals, from a real dataset. It demonstrates how to use the signal processing functions an visualize the results.

See: https://elephant--661.org.readthedocs.build/en/661/tutorials/signal_processing.html

The dataset used for this tutorial is added to elephant-data GIN repo in this corresponding PR:

https://gin.g-node.org/NeuralEnsemble/elephant-data/pulls/5

• Before merging this PR, the data path in the notebook needs to be changed from the local path to download the data from elephant data gin repo.

[coveralls]

coverage: 88.303%. remained the same
when pulling b749529 on INM-6:enh/tutorial_signal_processing
into febc7ce on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

Data

• remove grip signal to reduce file size and only keep LFP
• remove spike trains

Tutorial

• do not use al functions from signal_processing, but come up with sensible LFP analysis
• create overview of tutorial at beginning of the notebook:
  • key learning
  • contents
  • ??