refactor participant and group data value initialization, transfer, and API

[alee]

This is an area that needs improvement, both documentation-wise and in terms of framework / API support

round started handlers must invoke initialize_data_values with the appropriate parameters in order to initialize data values. If any errors occur in data value initialization, subsequent rounds will fail to run / load properly. This should be declarative instead, defined via the ExperimentConfiguration / RoundConfiguration objects. We can work to come up with a scheme for declaring how participant and data values can be transferred from round to round, and whether or not a round should transfer data values across or not.

Participants should create data values as they participate and generate data, not created at the start of the round and then updated when they generate data. For instance, the boundary effects experiment creates a new harvest decision data value every time a participant selects a harvest decision. The one that "sticks" is their most recent one with is_active=True.

The creation of and transfer of essential group data values has to occur when a round begins, so this may stay the same.