Generate Visit ID in ZEIR ETL

[csc5k]

We need to add a column to the ZEIR encounters ETL to generate a visit ID to use as a wrapper around encounters that occurred during the same visit.

The PATH team approved the following three data elements as identifying a visit: ZEIR ID, encounter date, facility ID. I.e. all rows with the same ZEIR ID, encounter date, and facility ID would share a single, unique visit ID.