Implement WLS

[jhorzek]

• implement tests for different wls/gls/... variants
• check handling of missing data in lavaan
• replace m2ll with wls in output
• check if we can somehow use automatic cross-validation.
• check how Huang selects models as AIC and BIC require likelihood
• check convergence criteria: wls is often very small and may require very strict convergence criteria. Maybe scale with sample size?
• create vignette discussing WLS
• add information about estimator to the SEM objects

[jhorzek]

Check "Model Selection in Structural Equation Modeling", Kristopher J. Preacher & Haley E. Yaremych