Memory

Addresses #50, #59, #66,

New features include:

* On-disk processing of xray data
* purely FFTs for correlations
* CXIdb compatibility
* test coverage
* many cleanups and improvements to `xray.py`

To possibly add before merge ( #38, #68):

* test coverage for polar masks
* parallel testing on Travis (tests are getting large)
* using individual shot energies for each 
* parallel processing of Shot interpolation