choose.trans returns a named list with two elements "trans"
and "inv", both functions which are each other's inverse. This is intended
to be stored as an attribute "Transform" with the resulting object
and used in plotting and reporting. All results will be on the transformed
scale. If the tr argument to choose.trans is a character
constant, the appropriate named list of two functions will be generated.
Possibilities are: "exp", "log", "logit", "pctlogit" (transforms percentages
by the logit), "sqrt", "sq" (square), "cll" (complementary log-minus-log),
"ll" (log-minus-log). If there is no match NULL is returned, which
will correspond to no transformation.