Allow user defined id field in MeasurementPeriods

[fpotier]

#745

[nitrosx]

@fpotier the code looks fine and it passes the test, so if we are in a hurry to get this feature in, I will be happy to approve it.
That said, I would like to propose two different approaches:

1. place the measurement period list in it's own collection with a proper schema, which will solve the unique id issue.
2. expand the concept of proposal with proposal type, parent proposal, and metadata (similar to dataset scientificMetadata) which will allow to create a hierarchical structure of proposals entities which has at the root your proposal and under each visit (aka measurement period)

Let's discuss

[fpotier]

@nitrosx Thanks, no hurry to merge it!
I'm okay with moving MeasurementPeriods to a dedicated collection but I'm not sure that the unique id issue would be solved.
Our user office uses a simple counter for each proposal (1..n), so in our case, we need uniqueness based on the proposal id. There can be an arbitrary number of period with id 1 as long as their proposal ids are different.