Add "do not import" option to import-export field map for RIS files

When setting up a field map for the RIS format in 6.x-2.0-rc1, there are a variety of fields that have a default of "none", including in particular, the keywords field (tagged KW). However, if the default field mapping is left alone with "none" showing, the keywords will in fact be imported into the biblio_keywords field. In my case, I wanted the option of NOT importing the keywords and was initially surprised to see that they had been imported even though I had not applied anything to their field map.

I can work around this problem easily by simply removing the keywords from the RIS import file, but it would be nice to have the option of explicitly choosing NOT to import one or more fields through use of the field map.

In terms of coding, it looks like there are a handful of tags that have special, or pre-set field mappings in the _biblio_ris_parse_line function of the biblio_ris.module that get tested before the default case arrives to map them according to the actual field map choices. I'm not sure the best place to insert this check, or how to set up the field map table to indicate a "do not import" choice, but I thought I'd make note of it as a minor feature request.

Phil.