New model for a purchase order reporting structure

[DJFresh66]

Are you a current Fivetran customer? Doug Jeffery, Modulous

What change(s) does this PR introduce? Adds required model for a purchase order reporting structure (follows the invoice model 'pattern')

Does this PR introduce a breaking change?

• [ ] Yes (please provide breaking change details below.)
• [x] No - New models only, existing models are untouched

Is this PR in response to a previously created Issue

• [x] Yes, Issue https://github.com/fivetran/dbt_xero/issues/21
• [ ] No

How did you test the PR changes?

• [ ] CircleCi
• [x] Other Using Fivetran transformations against our live Xero synch, manual tests in SQL to validate the volume and contents of the reporting table versus the source tables and the raw data. Also created a detailed download CSV and summary CSV which was QA'd against Xero by my internal customer (our CFO)

Select which warehouse(s) were used to test the PR

• [ ] BigQuery
• [ ] Redshift
• [x] Snowflake
• [ ] Postgres
• [ ] Databricks
• [ ] Other (provide details below)

Provide an emoji that best describes your current mood :sunglasses: Feedback

We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your feedback on our existing dbt packages or what you'd like to see next.

[DJFresh66]

Hi @fivetran-joemarkiewicz sorry for the delay getting back to you, I should be able to add the csvs as requested this week, cheers