indicators: add R018 Single bid received

[yolile]

Methodology:

Required OCDS fields: tender/procurementMethod, (tender/numberOfTenderers, OR tender/tenderers/id OR bids/details/tenderers/id)

Calculation method: For tenders i where tender/procurementMethod = 'open',

R018i=1 if tender/numberOfTenderersi=1

[yolile]

@jpmckinney comment: Can alternatively count the length of the array instead of checking for id

[yolile]

I guess we could also use bids/statistics/value when bids/statistics/measure = 'bids'

[jpmckinney]

Typically our approach is for the indicators command to only look in one place, and for the prepare command to fill in that place based on alternative fields. We do it this way, because having indicators try 5 different things is way too hard to reason about, when the indicator isn't as simple as R018. You can log alternatives in #86

For the first version, I would just use whatever seems like the best (most correct) option for Ecuador.

In the future, the best option really depends on how many datasets of interest use lots.

[yolile]

For the first version, I would just use whatever seems like the best (most correct) option for Ecuador.

Ok so, when tender/tenderers is present they always set tender/numberOfTenderes but they don't always set bids, so we can use tender/numberOfTenders.