Victor Purpura's distance.

Needs some more documentation, though.
Documentation of VP distance function.
Restructuring tests for spike train metrics.

Extracting common tests to its own class.
Van Rossum distance.

Needs some more testing, documentation and maybe some refactoring.
Added test for van Rossum distance.
Rename pairwise_max_idx_of_smaller_item(...) ...

... to searchsorted_pairwise(...).
Shortening searchsorted_pairwise(...) code.
Documentation of van Rossum distance.
Documenting _searchsorted_pairwise
Correction of van Rossum distance.

- Square root was missing.
- Did not work with one spike train being empty.
Test spike train metrics with empty spike trains.
Normalization documentation for van Rossum dist.
Make normalization match the documentation.

(van Rossum distance)
Convolution function for spike trains.

Still needs documentation.
Laplace kernel.
Returns discretization bins in st_convolve.
Use numpy assert function for comparing arrays.
Causal decaying exponential kernel.
Use class instead of kwargs for kernel params.

This will result in cleaner code as it saves us from passing down the
kwargs in addition to the kernel.
Rescale in histogram to be on the save side.