h1 (resp. calculate_frequencies) is too slow

[janpipek]

Actually, ~10 times slower than np.histogram. It has its reasons but (being more general) but in the simple case, we should not add any overhead.

Suggestion: when parameters allow, fall back to np variant.

[janpipek]

Related: #48

[janpipek]

For regular binning, it is possible to use this:

contents = linspace(...)
indexes = (data - low) / (high - low) # data is an array
numpy.add.at(contents, indexes, 1)

As suggested by Jim Pivarski.