Fixes bug where extract does not include warmup

Closes #101.
Remove model's temporary files when no longer in use

Delete the temporary directory holding the compiled model when there are no more
references to the model in Python (e.g., when the program terminates).

Adds associated tests (and cleans up pickling tests).

Closes #99.
Indicate missing type in Cython extraction functions
Fix bug where thinned chains were not extracted

Closes #102.