Expose parameters for handling NA values

[mpadge]

Extending from #2, another useful pre-processing step would be to enable user-control over handling of NA values. Current routines simply assume there are none, and error if any are encountered. Many stats routines have useNA or na.rm-type arguments, and there are also the stand-alone functions like na.omit. Exposing at least some minimal options which default to something like na.omit would greatly improve usability, through enabling NAs to be processed rather than current uninformative error. Even better would be to enable domain-specific options along the lines of zoo::na.approx(). Thanks!

[sevvandi]

Thanks again Mark. Really helpful. I removed NA rows after giving a message.