Implement bulk imports and edits for SLM models

[Hedde]

It seems relationships can only be imported based on their numeral primary key (which might makes sense as the name does not currently have a unique constraint)

[wkoot]

Also, the columns (field names) need to be lowercased - different from the export output

[rudradevpal]

was this issue fixed? I am still not able to do bulk import

[wkoot]

This has not been fixed yet, but we are accepting PRs ;)

[wkoot]

Implementation by @erichester76 posted in #44, we'll move this issue forward