Add parameter for missing value to trait importer (wide table)

[iimog]

When importing trait entries in wide table format it should be possible to skip missing entries. Currently this is not possible as an empty field raises an error. All other values are used as they are and imported into the db. A --skip-values parameter would be useful. If unset empty fields could be skipped if set all fields with that value are skipped. Examples: --skip-values NA or --skip-values '?'