Add rbind and + methods for mshapviz objects

[jmaspons]

No description provided.

[mayer79]

Thx for the contribution. I will check the details later, but I like it.

So far, a "mshapviz" object could indeed include any set of "shapviz" objects, even if column names were different. This was intentional, but I am about to change to a stricter logic: At least all column names should agree upon construction of the "mshapviz" object. This seems reasonable since "mshapviz" objects are typically used for subgroup analyses or multiclass models (or models with identical predictors).

This more strict definition of "mshapviz" will help to simplify certain plots, e.g., the already merged dodged bar plots.

I am working on a different branch on this, and it will help to simplify the checks in your PR.

[mayer79]

Thanks a lot!