Add functionality: select NA chars

[ehwenk]

In many studies there are "filled characters" used to represent NA. Currently these have to be converted to NA's in the custom_R_code using `~ na_if(.x, "character").

This function has gotten fussier with dplyr 1.1.0, no longer allowing mass substitutions across a mix of character and numeric columns. It might be better to specify within the metadata.yml file specific characters that are NA's for a given study.

This would be included as another optional field in metadata$dataset, probably appearing right before the custom_R_code.

[dfalster]

Moved to traitecoevo/traits.build#12