clean up stringsAsFactors

[johnlaing]

[not urgent]

To date we have relied on the default data.frame behavior of stringsAsFactors=TRUE. Sometimes this is appropriate, other times not. We should review all the relevant functions and attempt to "do the right thing" with respect to factors.

[eddelbuettel]

We should shoot the breeze a little.

I am actually not totally sure what it is we want. Blanket stringsAsFactor=FALSE may be too harsh. Adding a new switch or option is also weird as it adds little over users setting options(stringsAsFactors=FALSE).

Not sure.