Document jittering of OD data in which origins and destinations are different

[Robinlovelace]

Sometimes origin zones are different from destinations. odjitter will still work in this case, but pre-processing is needed.

• Description of the data
• Example input dataset
• Reproducible example
• Tests?

[Robinlovelace]

Heads-up @dabreegster we have a specific example of this. Plan: implement a reproducible fix quick on the R side and then think about support on the Rust side later maybe...

[dabreegster]

We have support for different origin/destination subpoints per zone, but the zones today are shared among both. The caller ought to be able to work around things here by renaming O_zonename and D_zonename or something like that. I'd vote against increasing complexity in odjitter to take distinct zones for O and D.