Convert an abbreviated month name to a numeric value
switch_abb(month)
a character string which will correspond to the numeric value of the input month, i.e. ("01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12")
a character string specifying one of the twelve months, i.e. ("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")