New correlation weights management

[heavyionanalysis]

New correlation weights management

[kreisl]

The observable and reference type are still there now it is named use_weights_. If use_weights_ is true the resulting correlation will be of type OBSERVABLE.
We now need to pass a second function which takes care of the weights. There are some premade ones for the cumulant methods.
For example:
v_2{2} = sqrt(c_2{2})
c_2{2} = <C_2> <> is event average

Edit:
In the case of my functions c2 = C_2 per event. So n2 is already divided out.
However It is still needed during the event average which is weighted by n2.
<C_2> = (sum_i^N C_2 n2) / (sum_i^N n2)

So n2 is equivalent to the weight / multiplicity m(m-1) the Q-vector
nd2 is supposed to be the same for differential cumulants, which make the distinction between particles of interest p and reference particles r. d2 is the corresponding numerator for differential cumulants.
it is a similar notation to: https://arxiv.org/pdf/1312.3572.pdf
Maybe we should adopt the same notation and link to this paper to make it easier to understand, what the purpose of these functions are.