Harmonize spelling and sorting of source data

[katrinleinweber]

Thank you for this pkg, @s87jackson :-)

I wondered whether some automated cleanup of codebook inconsistencies would be sensible? Presumably, no user wants to have slightly diverging category labels or unordered months?

This MR hard-codes such cleanups to the effect of for example:

CRAN release 1.1.0	this MR

Possible alternatives

1. (a) parameter(s) in front of any cleanup
2. dedicated helper functions
3. only a vignette with examples
4. Asking FARS authors whether this can be cleaned up in source data?