Add a function to discretise spiketimes

[Kleinjohann]

Adds a function to conversion.py which takes a SpikeTrain or a list of SpikeTrains as input and returns copies with discretised times.
This is useful for using spike_train_synchrony.Synchrotool with artificial spike trains which do not have a sampling rate, e.g. for Monte Carlo significance computations. It applies a sampling rate to those spike trains to make them resemble experimental spike trains recorded in a setup with an intrinsic sampling rate.

[coveralls]

Coverage increased (+0.007%) to 88.622% when pulling f4fa0db on INM-6:feature/discretise_spiketrains into d0d3920 on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

Thanks for contributing to elephant.
The proposed changes are optional, as they are not critical to functionality.

Unless otherwise declared, the acknowledged authors of this PR are:
kleinjohann

[Kleinjohann]

Thanks for contributing to elephant. The proposed changes are optional, as they are not critical to functionality.

Unless otherwise declared, the acknowledged authors of this PR are: kleinjohann

Looks good to me, thanks for the suggested improvements, I accepted and committed them.