checking S3 generic/method consistency

[russbiggs]

The R check is currently failing due to the way we are extending methods such as as.data.frame and plot. This results in a R CMD check warning which prevents the check from passing:

checking S3 generic/method consistency ... WARNING

The guidance is to follow this guide:

 See section ‘Generic functions and methods’ in the ‘Writing R
  Extensions’ manual.

We may need to rethink how we are exposing these methods to ensure the CMD check passes

[russbiggs]

@caparker can you take a look at this and maybe suggest some options?