add accumulate function to nb.Budget

Add possibility to overwrite function that is used for generating the total noise trace in an nb.Budget. This is done by introducing the accumulate class variable, which is set to the function to be used.

Stylistic question: uses direct assignment of function to accumulate and then type(self).accumulate to call this function. Considered using @staticmethod, but I don't know of a way to make the overwrite as clear and succinct then.