This PR is a revision of the existing Trials objects. Several improvements are implemented:

Functionality

1. Added additional methods to retrieve spike train or analog signal data across the trial repetitions (get_spiketrains_trial_by_trial and get_analogsignals_trial_by_trial).
2. Moved redundant code to the base class Trials, for improved maintenance.
3. Refactored unit tests to include content checks. The tests are based on a base unit test class that is shared across different test cases and provides custom assertions to compare Neo objects.
4. Changed name convention of arguments to identify trials as indexes (trial_index), instead of the previous trial_id or trial_number.

Documentation

1. Module docstring text improved.
2. General revision of all classes and methods docstrings, to harmonize style and add detailed content of the API.
3. Added usage examples to TrialsFromBlock and TrialsFromList.

TODOs

• Sphinx configuration to handle links to Neo classes in the documentation.