[ENH] add trials tutorial notebook

[Moritz-Alexander-Kern]

This PR adds a new Jupyter Notebook tutorial for the elephant.trials module, demonstrating how to handle trial data in neuroscience experiments using Elephant. The notebook covers the usage of TrialsFromBlock and TrialsFromLists classes, providing detailed examples and explanations.

Changes

• added trials.ipynb notebook:
• updated documentation to include the new tutorial

See also: https://elephant--639.org.readthedocs.build/en/639/tutorials/trials.html

[coveralls]

coverage: 88.35%. remained the same
when pulling 31dcd19 on INM-6:enh_trials_tutorial
into 0984e19 on NeuralEnsemble:master.