The main purpose of this flag is as a quick test to see if the
altitude grid-spacing on the user-supplied profiles
is sufficiently fine to support the assumptions
used in equating atmospheric paths to equivalent homogeneous paths for
the Curtis-Godson approximation and Planck-function variation
(see also BFX flag for Planck-variation).
If the results with/without the LAY flag are significantly different
then the layering is too coarse.
If the user supplies a profile grid of N levels (specified by
the first profile in the first file of the
*ATM section), then this routine
add an extra N-1 levels at the intermediate points, specified in
altitude on ln(p), according to which type of grid is used.
The PRF Flag can be used to write out
the new profiles that are actually used