EB STD calculation

I don't know if you think the design is ok. In the end, since calculating the std was so similar to calculating the com, I decided to put it directly in calc_coms with a boolean parameter calc_std. It's not very pretty since I needed to add several if calc_std in the function... But otherwise creating a totally new function would end up in copying almost all of calc_coms. But that's up to you if you think that's better.