umx_scale
applies scale to the columns of a data.frame. By default it scales all numeric columns,
and is smart enough to skip non-scalable columns (strings, factors, etc.).
You can also select which columns to convert.
This is useful when you want to avoid numeric columns which are actually factors.
note: By default, the attributes which scale adds ("scaled:center" and
"scaled:scale" removed to leave nice numeric columns. Set attr= TRUE
to preserve these.