use data model from #18 as intermediate data structure

[tillmo]

split of the code in purchase_invoice.py, function parse_invoice_json into two functions:

1. the value in the json field is used to create data according to the data model in #18 is an intermediate data structure
2. from this, an ERPNext invoice is generated.
   The purpose is that the first function can be used to test the invoice AI, by comparing the result with the value of the json1 field.

[Andrewed95]

Done