[ENH] Integrate trials object with Fano factor

[Moritz-Alexander-Kern]

This pull request introduces an enhancement to Fano factor using the the elephant.trials module to represent trials. This change is part of a greater effort to provide a more structured and unified approach for handling trial data in Elephant.

Trial representations

• trial object: The new trial_object is now used as the primary input structure for representing trials. This object handles trial data, see https://elephant.readthedocs.io/en/latest/reference/trials.html .

• list of lists: The previous implementation relied on a list to represent one trial.

Changes

• fanofactor no handles trial object
• added tests to test Trial object input