psem takes a list of structural equations, which can be model objects
of classes: lm, glm, gls, pgls, Sarlm, lme, glmmPQL, lmerMod,
lmerModLmerTest, glmerMod, glmmTMB, gam.
It also takes objects of class formula, formula.cerror, corresponding
to additional variables to be included in the tests of directed separation
(X ~ 1) or correlated errors (X1 %~~% X2).
The function optionally accepts data objects of classes: matrix,
data.frame, SpatialPointsDataFrame, comparative.data, or these are derived
internally from the structural equations.