prepare: Fill in from alternative fields

[jpmckinney]

Some ideas that can be implemented depending on which datasets we want to run against:

R003

• Change to use tender/datePublished (OCDS 1.2) and prepopulate it from tender/tenderPeriod/startDate

[yolile]

R018 - Single bid received

• Fill tender/numberOfTenderers with: len(tender/tenderers) or len(bids/details/tenderers) or bids/statistics/value when bids/statistics/measure = 'bids'