Feature request: Support mixed model feature extraction

[adibender]

Goal is to have random effect estimates of a mixed model as features in the model

Should be relatively straight forward

1. Transform data to long format (i guess there is something in tf package)
2. Fit Mixed model with where the functional feature is the outcome and id the input using nlme or lme4 (formula would need to be supplied, but usually we want random intercept and random slope)
3. "predict" the random effects (intercept + slope) for each id
4. return