WIP : first pass at auto-circle finding

 - implement separated 1D correlation between rows/columns and
   it's mirror
 - find maximum in this correlation to estimate symmetry center for each
   slice
 - use Hough transform logic to vote on location of symmetry plane for image
BUG/TST : added tests for center finding

 - fixed bug related to square images
MNT: Added scikit-image to conda install of travis yaml file
BUG : return a tuple as the doc claims
ENH : change what _corr_ax1 returns

return the histograms rather than just the most likely
mirror plane location.
DOC : added name of return value for wrapping reasons
Merge pull request #103 from tacaswell/powder_center_corr

Powder center corr