Support for categorical attributes

[meakbiyik]

Something from SAOM: when we test a statistic over a categorical (e.g. character) attribute, SAOM creates multiple binary attributes from that and tests them altogether, with some good naming scheme. As far as I remember, it looked like this (though I might be mistaken):

ego.gender.male -> ...
ego.gender.female -> ...

We can of course achieve this by manually one-hot-encoding these variables, but this would result in very long formulas that might be harder to oversee, and lots of preprocessing work. I am also not sure whether this can be optimized if done on goldfish's side. In any case, I would really like to be able to add such an effect:

formula <- dep_var ~ ego_categorical(days_of_week)

This also would be mighty helpful with global attributes! (as mentioned in #25 )

[jhollway]

This could be handled in preprocessing where necessary, similar to how it is managed in network_reg() in {migraph}?