Require mapping for any field

Historically, the user only had to map fields with a required validator, in which case any record missing a value would raise an error. Requiring mapping was tightly coupled with requiring values. But in some instances, you might want to require that a column appear in the final dataset without necessarily requiring a value for every record.

To support this, we have added a requireMapping boolean to the field object. When set to true, the user will not be able to progress to the data review screen unless this field has been matched to a column in the input file. More info is in the docs.