add quasi-likelihood

I added an attribute "quasi" and a method "compute_dispersion", so that the dispersion is now used when reporting std errors (see get_stderr() in IRLS.cc).
I tested the results with R and obtain the same values (one day if I have time I'll add some automatic tests to the repo.)

I also removed useless arguments in some functions (the data structure "gsl_vector" contains its size, no need to pass it along).