Block decomposition: add specialized normalization algorithm for purely backward models

If the model is purely backward, determine whether all original equations have
a single contemporaneous endogenous on the LHS. If this is the case, then first
try a normalization by enforcing that each original equation is matched with
the endogenous on the LHS.

This helps with the simulation of purely backward models, where equations are
renormalized with mfs=3, since it produces a simpler system to be recursively
evaluated/solved.