Added gen_batches() to utils

This generator is similar to gen_even_slices(), but takes a slice length
as input instead of a number of slices.
Changed pairwise_distances_argmin API

- Chunking is now performed passing chunk size instead of chunk number.
  Default value has been set to 500 after some benchmarking.
- Dropped the "return_distances" keyword (useless)

Tests have been adapted.