Have a Tree constructor with nu

[anilbey]

Right now it is set explicitly after the constructor is called in here https://github.com/anilbey/sc-dna/blob/19760ae2e9a606d0a4c53fc384310442dfeff84b/src/infer_trees.cpp#L189 here https://github.com/anilbey/sc-dna/blob/19760ae2e9a606d0a4c53fc384310442dfeff84b/simulations/run_simulations.cpp#L93 and here https://github.com/anilbey/sc-dna/blob/19760ae2e9a606d0a4c53fc384310442dfeff84b/simulations/Simulation.h#L84

Have a constructor overload for the overdispersed case and use it instead.