Added the feature to import fixed strain-plasmids

* This feature imports data for fixed strain-plasmids
   * There are plasmids associated with strains that are not real plasmids or link to real plasmids
* The data has been fixed manually and is available [here](https://github.com/dictyBase/migration-data/blob/master/plasmid/Strain_plasmid.tsv).

*Note: Remove header line for import*