Add the ability to register callbacks for errors that occur on the
asynchronous variable sampling thread. Implement __del__ and call
close in case the user forgot. Join the asynchronous sampling thread
in close before returning.
Refs #365
Namespace packages are a mechanism for splitting a single Python
package across multiple directories on disk. With the addition of
$(TRICK_HOME)/pymods/trick, there now exists a package named 'trick' at
$(TRICK_HOME)/pymods and in each SIM_* directory. This change allows sims
to import modules from both locations.
Refs #365
