Save and reload Person Id map

[PhilAppleby]

Aimed initially at PHS data transformation

Previously processed person-ids and the values generated and mapped to should be saved to allow data increments.

New person ids should be saved when received in demographic data increments, existing person ids should be checked when transforming data to OMOP. Currently data sets have to be self contained with person-ids flagged as invalid if not found in supplied demographic data.