Stock for D8: Create Low level API

Problem/Motivation

Develop a low level API for working with the database schema.

Proposed resolution

The following is not a complete list but a place holder to add to over time.

Is product In Stock ( Product ID, Location List, Check all locations ) - If Check all locations is FALSE will return TRUE on the first location with stock. Otherwise will combine the stock from all locations and return TRUE if positive (this is for support of negative levels)

Product Available Stock ( Product ID, Location List) - the total of stock from all the locations listed.