Computes the potential temperature of seawater relative to a
chosen reference pressure following Fofonoff and Millard (1983).
The potential temperature \(\theta\) is the temperature that a
water parcel would have if were moved adiabatically to another pressure
level Pref. Typically, the potential temperature is referenced
to the surface (\(Pref=0\)). The potential teperature depends on
the original salinity S, in-situ temperature T
and pressure P.
This routine is essentially a wrapper for the swTheta
routine of the 'oce' package. Unlike the latter, pressure units here
are given in bars and method="unesco" is prescribed.