Remove variable from the formula if the SD of the variable is zero

[bniebuhr]

We should have a feature on fit_net_logit and fit_net_clogit for removing variables of the formula (and providing a warning) if a variable has SD zero.
Also, it would require us to check the presence of all variables afterwards in bag_models and adding NA coefficients to those variables from the models where they were removed.

[bniebuhr]

This will also require to check for NA coefficients when making predictions etc, so they do not alter the results and functionalities

[bniebuhr]

This was now partially coded in d940963 and 717d266. Needs to be further tested before the issue is closed.