Add a building block to the considered equation

Usage

add_equation(equation, item, coeff = 1, fixed = TRUE, loading = NULL)

equation: the equation
item: the block of the state array that will be linked to the observation corresponding to this equation through the specified loading and coefficient
coeff: the value of the coefficient associated to the block of latent variables defined by item.
fixed: logical that triggers estimation of coeff (FALSE) or fixes it (TRUE) to a pre-specified value
loading: the loading that links the block to the observation