Don't export as.codedata

[eribul]

• [x] Don't export as.codedata, and instead do the preprocessing/checking of codedata within the codify function instead of suggesting that the user use as.codedata
• [x] If you're very confident that performance when keeping the codedata the same and trying many different datasets is important, you could add a function called classcodes_prepare or prepare_classcodes that does the conversion to data.table and sets the indices, and could describe that in the details section of the documentation. But I'd want to understand why that's a typical use case.

[eribul]

Review: I agree and have integrated as.codedata into codify! I have simultaneusly loosen the expectation that the codedata must have columns named id, code and code_date. INstead, the names of corresponding columns should now be specified as arguments in relevant functions.