Line length.
Rename kernel in metrics to smoothing_filter.

The term kernel will be used to denote something slightly differnt.
The same distinction as in
Paiva, A. R. C., Park, I., & Príncipe, J. C. (2009). A comparison of
binless spike train measures. Neural Computing and Applications, 19(3),
405–419. doi:10.1007/s00521-009-0307-6
will be used.
Fixing units in a cs_dist test case.
Schreiber et al similarity measure.
schreiber_similarity() documentation.
Refactoring

Allowing to use more efficient algorithm with schreiber_similarity in
certain cases.
Refactoring van_rossum_dist.
Normalization of kernels
Allow different kernels in victor_purpura_dist.
schreiber_similarity for multiple trains at once.