data .py getter supports laziness in first dimension
use map term instead of imap
json data descriptor is lazy in the first dimension
ordered index works when on subindices
concat/stack descriptors are lazy in first dimension
Stack.py[0] works and tested
data: close file if result is not iterator
pep8
Merge pull request #270 from mrocklin/lazy-data

Data layer supports streaming access with `.py` getitem