[Developers] admb threading

[dave fournier]

I guess you didn't bother wasting your time looking at my example for 
dvariables.

The big problem with these objects is not some trivial locking in the 
constructor etc.
It comes from the fact that the data structure which store all the 
information for
multiple levels of reverse AD are not thread safe. If you are doing 
df1b2variable
arithmetic each thread needs its own copy of these.