histEntropy: additive entropy for normalization and similarity

[andymilne]

Consider allowing any number of vector arguments such that the entropy is calculated for their sum but divided by the sum of entropies of each vector. This could serve as a normalization different to the one currently used, but possibly more relevant to the purpose at hand.

It is also possible that entropy differences could be used to calculate (replace) cosine similarity to determine the similarity of any two spectra.