Subset function that preserves column attributes.
subsetp(x, ..., droplevels = TRUE)An object similar to x containing just the selected elements.
In the case of a data.frame, attributes of columns (such as
label and units) are preserved.
An object to be subsetted (usually a data.frame).
Further arguments passed to subset.
If TRUE (the default), then unused factor levels are dropped (see droplevels).