internal/design adjustments in discretemod

[joep]

a few notes on discrete models, while looking how nested logit fits in

* nparams
  aic, bic, ... calculations need the number of params self.df_model+self.model.J-1,
  I need it also for a default starting values and other things in ru2nmnl
  If we set nparams in the model, then bic, aic don't need to be special cased for MNL. We could use df_model as default if nparams doesn't exist in the model

* in MNL.J and .K are not very informative when reading, nchoices and nexog would be more informative
  in ru2nmnl, I started to use nchoices, I don't think I have any nexog.

* llnull: I don't think this will work in nested logit, and maybe not in clogit. Unfortunaltely the the arguments to model.__class__.__init__ are more complicated than the generic (endog, exog)
  --> possibility to override in model or in a Results subclass ?

* llnull: do we need the null model for anything else ? currently null model is not attached

* Rsquare: for MNL type models, SAS (proc MDC) has six definitions additional to McFadden, they are all easy to calculate given llnull

extras not checked yet
Confusion/classification matrix ?

(I expect to add more as I work my way through this.)