Add linear pdf using RooPolynomial
Define a linear PDF using a RooPolynominal of order 1. The current implementation takes one parameter a1 and defines a pdf based on the polynomial 1 + a1 x.
It might be more elegant to implement a general class for any order polynomial but that would require optional parameters and I don't know if the framework can deal with that. So for now I've implemented a linear pdf with a fixed number of parameters, which is enough for my use case.