Starting with 2.2, the HYCOM archive files no longer include density.  This 
is because T&S define density via the equation of state.  However, this means
that the archive file must include information about the actual equation of
state.  So "k  dens" for montg1 (the first record in the archive file) are
actually sigver and thbase, where sigver is a small integer identifying the
equation of state (taken from a parameter defined in stmt_fns.h).

So far we have (odd for sigma-0, even for sigma-2):

sigver =  1  ---  7-term sigma-0
sigver =  2  ---  7-term sigma-2
sigver =  3  ---  9-term sigma-0
sigver =  4  ---  9-term sigma-2