Major update that improves support for formulas specification

[stefvanbuuren]

• reintroduces the square predictorMatrix
• defines conversion functions p2f(), p2c(), f2p(), n2b(), b2n()
• defines validate.blocks(), validate.predictorMatrix()
• extends edit.setup() to formulas and blots
• for reading ease, use ~ 1 for the empty predictor set instead of ~ 0
• does not automatically set method = "" for variables that are not imputed (NOTE: DECISION REVERTED. SEE BELOW)
• as far as possible, changes the leading argument to formulas (instead of blocks or predictorMatrix)
• adds function typecodes() in sampler() to reduce multiple predictorMatrix lines to one (support for multivariate imputation methods)
• implement new logic in samper.univ()
• outcomments some tests that depend on hard-coded parameter estimates
• sharpens test for equality between predictorMatrix and formulas specifications