add more robust filter_dv testing and thought

[dpastoor]

need to determine if the current method (returning x if class not recognized) is viable or should make a data.frame generic and then throw error for rest (eg do we want this to proceed silently if someone passes in a matrix or otherwise?

filter_dv also needs generic testing for non-classed passed through (eg passing data frame)

[ronkeizer]

sure, adding more tests is fine. 'matrix' class can be converted into data.frame, we can throw a warning that they should not pass matrices, but should proceed anyhow.

feel free to improve on my updates of the filter_dv() function, I just needed this fixed quickly for a colleague that needed a vpc a.s.a.p.