In procedure "disvert", the values of "sig" are now chosen according
to the value of "vert_sampling". "ok_strato" no longer appears in
"disvert". The analytical formula that was used when "ok_strato" was
true in "disvert" is now selected when "vert_sampling" is set to
"strato". Motivation: we can now read values of "sig" instead of
using this analytical formula, even when "ok_strato" is true.
Default value of "vert_sampling" with llm=39 gives the vertical
distribution of CMIP5 simulations.