extending `getSumWgts` to get better control on the use of the weights

The code of getSumWgts had been initially thought in the context of jet analyses, where the sum of the generator-level weights was an undisputed choice for the normalisation of the MC n-tuples, with no need to cut off on any generator-level weights.

However, in the context of Drell-Yan analyses, this simple strategy happened not to be valid anymore. The problem is discussed in #88. This MR extends the code of getSumWgts to provide more control on the handling of the weights. It provides three modes: the sum of the weights, the sign of the weight only, the count of events.