Add 'age' column 'Units' into participants.tsv

[yarikoptic]

bids-specification already allows for overload of the unites.

• beware that full definition record needs to be provided , not only Units.

Relevant links:

• [ENH] Recommend controlled vocabulary for age Units, clarify that it can be overloaded bids-standard/bids-specification#2400
• columns.yml: unclear difference in schema in defining within record or within "definition" sub-record bids-standard/bids-specification#2045