Chunking on both arrays in euclidean_distances_argmin

Chunking is now possible on both arrays in euclidean_distances_argmin.
The default chunking scheme has been tested for various array sizes, and
seem to be at least as fast as euclidean_distances + argmin invocation,
while using a lot less memory (around 50 times).

At the same time, check_pairwise_arrays has been improved to not copy
arrays that have a np.float dtype.