Add info on how to run doctests in temporary directory

[pdxjohnny]

The following code goes in doctest_global_setup in docs/conf.py

import os
import shutil
import atexit
import tempfile
import functools

# Create a temporary directory for test to run in
TEMPDIR = tempfile.mkdtemp()
# Remove it when the test exits
atexit.register(functools.partial(shutil.rmtree, TEMPDIR))
# Change the current working directory to the temporary directory
os.chdir(TEMPDIR)