Flexible nonlinear extension of spatial autoregressive (SAR),
spatial error (SEM), and spatial autoregressive with autoregressive
disturbances (SARAR) models with multiple regression engines
(generalized additive models ('mgcv'), gradient boosting ('mboost'),
multivariate adaptive regression splines ('earth'), and 'xgboost')
and two families of spatial-parameter estimators: maximum likelihood
and the determinant-free Closed-Form Estimator of Smirnov (2020)