this function computes the change in phase delay rate caused by
the wet and dry components of the earth's neutral atmosphere.
this routine uses pfeiffer's anlytic formula to get the rate of
atmospheric delay as a function of zenith angle rate;  it is
it is therefore inconsistent with eatmdl, which uses chao's table.
wetz - wet zenith correction (sec.)
dryz - dry zenith correction (sec.)
zen  - zenith angle (rad)
dwetz- derivative of wet zenith correction (sec/sec)
ddryz- derivative of dry zenith correction (sec/sec)
zendot- derivative of zenith angle  (rad/sec)