streamline documentation of `mode` and `engine` in models

[EmilHvitfeldt]

working on #1040 was quite a puzzle.

Not high priority, but I think there is a better way to handle this. In an ideal world, there would be an @inherits that JUST handled mode and engine.

i would like it to look like the following:

@inheritFrom base-reg

@inheritFrom base-class-reg

@inheritFrom base-reg-cens

@inheritFrom base-class-cens

@inheritFrom base-class-reg-cens

Or some other type of roxygen magic.

@param engine is defined 9 times and @param mode is defined 16 times. But should be no more than 5 atm.