Importer: Don't require columns for non-mandatory fields

[tbar0970]

Running an import I got

Your file must contain a gender column
Your file must contain a work_tel column
Your file must contain a address_street column
Your file must contain a address_suburb column
Your file must contain a address_state column
Your file must contain a address_postcode column

Considering gender can be "unknown" and work-tel and address are not mandatory fields, these columns should not be compulsory.

Extra point: If I'm doing an import where every row should match an existing person, I currently have to add a blank "status" and "age bracket" column. It would be good to stop this being necessary. Perhaps just allow the column to be absent and throw an error further down the chain if trying to createa new person without status etc.