Rename sample.py to sampling.py

To avoid any confusing with function pymc.sample, which is the imported
from model pymc.sample in __init__.py

Author: kyleam

pymc/__init__.py

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