tadaadata/tadaatoolbox: Housekeeping

Changes

• Fix error in SEM calculation in tadaa_one_sample for t-tests
• Make sure tadaa_-test functions use tadaatoolbox::pval_string instead of the pixiedust version
• Remove recoded leist var from ngo, as it should be computed from leistung by students.
• Move cowplot from Imports to Suggests because whe only need it in one function, sometimes.
• More compact table output in tadaa_nom and tadaa_ord.

• [tadaa_]likertize is removed. Use sjmisc::split_var.
• labels_to_factor is removed because various as_factors exist.

• Switch to the markdown flavour of roxygen2