Modify external function interface to pass ADF object

[nusbaume]

Currently in the ADF a set of specific variables is passed to each external script. However, this makes certain parts of the internal ADF code quite complex, and makes the python ADF/script interface overly rigid and potentially difficult to work with.

To alleviate this, it might be better to pass either the main ADF object itself, or another script-specific object, into each external script. This would make the ADF/script interface simple and common to essentially all scripts, give the scripts access to every variable that the ADF holds (that isn't private), and would make the ADF lib code itself a lot cleaner. The downside is that a script developer would need to learn how to use this object, but with good documentation that learning effort will hopefully be minimal.

For the discussion that helped initiate this issue, please see: #65

[andrewgettelman]

Thanks @nusbaume . I support this idea, but want to make sure the object is easy to use by the developers. So maybe we need an example of how it might be used by a plotting routine. It doesn't have to be implemented to do this, but how would I write a simple plot routine to use the data? Maybe a short session for those interested would be useful. I'm happy to help.