Add attribute to ArviZ objects about model type

If we add the type of model a fit came from (i.e. default NB, default multinomial), we could check it in diagnostics/viz functions and proceed accordingly.

Start by locating where ArviZ objects are created and where diagnostics and visualization functions consume them. Determine how default NB and default multinomial model types should be represented and verified; done when those functions can reliably proceed according to the stored model type.

Written by the indexing model from the issue text.