Clean up generics

[gowerc]

Currently the generics are a mix of implicit and explicit and are inconsistent in terms of how they are exported and documented. Would be good to clean this up.

Taking inspiration from Bioconductor I think we should define explicit generics rather than being reliant on the implicitly generated ones. In particular for things like as.data.frame care is needed on which arguments should be exposed for multiple dispatch (again taking Bioconductor as an example)

[gowerc]

This is also connected to #177 . My current thinking is we should implement these as S3 generics (to avoid these collisions and inconsistencies) that in turn dispatch to the as method to handle the actual s4 conversions.

[gowerc]

Ok 1 major issue with the as() approach is that you can't specify any arguments to the coercion function

[danielinteractive]

Yes true - if you need any other arguments then it is cleaner to have a dedicated constructor function, think ObjectFromOther(other, opts)