Conditional / dependent source distributions

[paulromano]

Description

I've had a few recent discussions about the desire to have a conditional or dependent source distribution where, e.g., the angular distribution could depend on the outcome of sampling the energy distribution. This is an option in MCNP via the DS card. I'm not sure what the interface would look like but thought I'd create this as a placeholder issue for further discussion.

Alternatives

You can kind of brute force this by creating a bunch of individual IndependentSource objects that collectively behave similar to a dependent source distribution. You can also, of course, write a CompiledSource to achieve this, but that doesn't really give a general user any ability.