Add NASIS `relationshipmaster` table snapshot dataset

[brownag]

It is useful to have access to relationship information for NASIS tables. Relationship info is a standard part of the SSURGO metadata, but accessing it for NASIS is a bit more tricky. We currently don't have a tabular version of the relationships available on NASIS metadata page

• Create web report for querying relationshipmaster for specific data model version
• Add snapshot of relationshipmaster table for current data model
• Add process/script for regenerating on future data model release
• Add function/convenience method get_NASIS_relationship() (or similar)