Fix and improve estimate standardization

[maltelueken]

Changes the way estimates are standardized:

• Now, three options are available: Unstandardized, mean-centered, standardized
• All variables are standardized before any other computations are made (including three way interactions)
• For listwise missing value deletion, only compute scaling based on complete cases; because listwise handling deletes incomplete cases, otherwise the the variables used by lavaan would not be correctly standardized anymore
• Adds a footnote indicating that estimates etc. are standardized
• Fixes a bug for calculating dummy variables with missing values