Fix multi-model dispatcher indexing and column order

[michaelzehetleitner]

Summary

• normalize optional scalar columns (e.g., st0, sz, sv, wx, wrt, wint) in
  every predictor and simulator before they are used or passed to C++
• rebuild theta*/thetaUpper*/thetaLower* columns defensively so sparse
  parameter sets (including nRatings == 1) no longer trigger undefined-column
  errors
• remove the previous NA-based column dropping so that callers can omit
  optional parameters without breaking downstream models

Testing

• pkgload::load_all(".")
• testthat::test_dir("tests/testthat")

[SeHellmann]

Does not really change behavior, but I am sure, the current version works as intended. So, not merged.