[ENH] time_histogram handles single `neo.SpikeTrain`

[Moritz-Alexander-Kern]

This pull request addresses issue #648 by updating the statistics.time_histogram function to accept a single neo.SpikeTrain object as input.

Previously, the function was limited to handling lists of spike trains, which led to incorrect results when a single spike train was provided. This enhancement ensures that the function now correctly computes the time histogram for both single and multiple spike trains.

Changes

• Modified the time_histogram function to handle a single neo.SpikeTrain input.
• The documentation has been updated to reflect this new capability. It now clearly states that the function can accept both a list of spike trains and a single spike train.
• Added a regression test to ensure that the functionality works as intended for both single and multiple spike trains.
• Added type hints to time_histogram
• Removed copy parameter when creating pq.Quantity object

[coveralls]

coverage: 88.257% (-0.1%) from 88.401% when pulling 9c06d0d02010e3f559fefcd87648f0a7401b28a9 on INM-6:fix/time_histogram_648 into 123ca040b11a5961643c4de080df89af9b1fcd24 on NeuralEnsemble:master.