Internal helper to fit a linear mixed-effects model,
following the formulation described in Laird and Ware (1982).
See lme for details.
Thiago de Paula Oliveira, thiago.paula.oliveira@alumni.usp.br
Laird, N. M.; Ware, J. H. (1982). Random-effects models for longitudinal data. Biometrics, 38, 963–974.
Pinheiro, J. C.; Bates, D. M. (1996). Unconstrained parametrizations for variance–covariance matrices. Statistics and Computing, 6, 289–296.
Pinheiro, J. C.; Bates, D. M. (2000). Mixed-effects models in S and S-PLUS. Springer.