review mod_date and add_date slots and current use in NMDC

[aclum]

I believe these fields originally derived from GOLD and are being populated by the GOLD to mongo ETL.

I would like to discuss if we want to keep these specific to GOLD as source of truth metadata or use them more generically (which is how they are currently described).

Minimum set of changes would to be to change the range from String to date

[aclum]

@mslarae13 @cmungall @turbomam @sujaypatil96