add sources for utilities and probabilities

Documenting the source of values may be necessary.

Source uncertainty: it would mainly described the source of the probability values (e.g. from literature, from expert judgements, from internal database/computations).
Source utility: it would be used to say where that data comes from. This can be also an identification from which database or dataset this data came from.

The issue names no files, tests, or entry points. First inspect where utilities and probabilities are represented, then clarify how source uncertainty and source origin should be recorded and exposed. Done means both source types have an agreed representation and the resulting behavior is covered by tests.

Written by the indexing model from the issue text.