Rename sample.py to sampling.py

This avoids the naming conflict between the module pymc.sample and the
function pymc.sample, which is imported in the top-level __init__.py.
The conflict prevented direct access to the module.

Author: kyleam

pymc/__init__.py

@@ -5,7 +5,7 @@
 from .math import *
 
 from .trace import *
-from .sample import *
+from .sampling import *
 from .step_methods import *
 from .tuning import *