The intention here is to make sure there is a CWRES column if applicable
so that the cwres variant functions can be used rather than the
cwresi variants.
supplement_cwres(x)a data frame
If CWRESI was tabled out, then a CWRES column is added. No change
is made in case there is already a CWRES column. This function is
called with every call to a cwresi variant, so, ideally, the user
can call this function once upon loading the data.