importer should check for existing style

[ischneider]

Before executing Importer.addToCatalog, a check should be made for an existing style and a new one computed if one exists. This usually won't happen but could as the 'unique' style is computed early in the process. If the import process is halted and another is begun, they will both have the same style name as the intermediate state is not persisted to the catalog.

[jodygarnett]

This issue should be migrated to JIRA as importer-core is no longer maintained in this repository.