If field is a sentineled field, a data.frame with two
columns. The first is a numeric version of the continuous values
from x. Its name is the value of field. The second is a
factor with levels representing the sentinel values. For all
non-missing values in the numeric vector, the respective value in the
factor is NA. If a value of x was not valid, the respective
row will be NA for the continuous and flag values.
If field is not a sentineled field, a data.frame with just x
is returned with a warning.