Document expected usage of default characteristic of scenarios, especially in use with clone()

[gidden]

Per our discussion in #97, it would be useful to have a documentation page that covers aspects of how versioning is done on model/scenario data. There are two main topics I see that could be covered at the moment.

First is the application of the default characteristic. How is this updated? What happens if there is no default version? How does this interact with clone()?

Second is the possible results from clone() with various arguments. E.g., what if I only give model, only scenario or both? What if model and scenario are the same as the copy-ee?

This could be pretty simple page enumerating all possible procedures with expected outcomes.

[khaeru]

Some of this also cropped up in #108, but wasn't fully addressed there.