this function computes the phase or group delay in seconds caused
by the wet and dry components of the earth's neutral atmosphere
wetz - wet zenith correction (sec.)
dryz - dry zenith correction (sec.)
zen  - zenith angle (rad.)
this model uses the mapping table presented in
jpl iom 391.3-352 may 25, 1971

rwp1 0.1 degree increment for wet correction (0.2-10.0 deg)
rdp1 0.1 degree increment for dry correction (0.2-10.0 deg)
rwp5 0.5 degree increment for wet correction (10.0-60.0 deg)
rdp5 0.5 degree increment for dry correction (10.0-60.0 deg)
rp5 0.5 degree increment for wet & dry correction (60.0-90.0 deg)