delta_ck and DRAT, but may not work yet
fix ptrm stats
bah!  now the first part is actually working
fix error in check(%)
more ptrm stats
DRATS, CDRAT, max_DEV all seem to work.  DRATS still has a possible problem of including different values in thellier_gui and in spd
DRATS_prime added, tests passing
mean_DRAT and mean_DRAT_prime