Extend DataSource class to better facilitate model versioning

[Flix6x]

We often have new beliefs data created by some model, like a forecasting or scheduling model. Sometimes models go through different versions, sometimes they are switched out altogether. Currently, our DataSource class only facilitates keeping track of which model data comes from, by creating a new DataSource with a different name. Originally, that DataSource name was intended to name which organisation or user sourced the data. I suggest we add a little more structure to support versioned models.

Here is a suggestion:

DataSource(name=<name of organisation running model>, model=<name of function defining model>, version=<model version>)

[nhoening]

If model and version are nullable, thus not required, I agree.