Implement WLS

[jhorzek]

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

[jhorzek]

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