- (__sub__) operator for histograms

[beojan]

Histograms can already be subtracted by adding -1 times the RHS. It would therefore be nice to have a - operator on histograms.
Personally I find this useful when comparing histograms (I tend to prefer plotting (h2 - h1) / h1 instead of h2/h1 since the sign lets you see the direction of movement easily).

[henryiii]

@HDembinski What do you think?