Closes #1320

[MichaelChirico]

adds rownames argument to setDF, per #1320; adds an error for the case that rownames is given of incorrect length.

[jangorecki]

Looks pretty fine. Some minor things that can be added is a more verbose error message on non-matching length - there could be info about it should match to nrow - and a check if rownames is character as I believe data.frames rownames must be a character(?).

edit: before PR be sure to install new version locally and run test.data.table(). I restart R session each time I install data.table just to be sure it won't cause issues.

[MichaelChirico]

@jangorecki thanks for the quick feedback. my first more-than-trivial commit on a project that's not mine :)

As to the error message, I think that error is handled "on the backend", e.g.:

DF <- data.frame(a=1:10,b=1:10)
setDF(DF, rownames = factor(LETTERS[1:10]))

gives me an error:

Error in setattr(x, "row.names", rownames) : 
  row names must be 'character' or 'integer', not 'integer'
Called from: setattr(x, "row.names", rownames)

More worrying is that there's no such "free lunch" with respect to duplicate names, so I think I'll add a check for that: setDF(DF, rownames = rep("a",10)) returns no error until the next time you try and display DF when it gives: Error in data.frame(a = c(" 1", " 2", " 3", " 4", " 5", " 6", " 7", " 8", : duplicate row.names: A