allow merging of spiketrainlists with arbitrary t_start and t_stop

[JuliaSprenger]

Currently times of spiketrain lists have to be identical for merging and an Error is raised in other cases. This should be extended to also work with non-identical times

I think a later one.

Originally posted by @apdavison in https://github.com/NeuralEnsemble/python-neo/pull/1000#r654517205