Fixes bug in cube_to_image function for case where no slicepos given
Removes dtype from cube_to_image summing case, introduces make_uniform_image to produce an image of ones (required in the test case) and introduces a test case for cube_to_image
combines functionality of make_empty_image and make_uniform_image and adds checkerboard option. Updates docstring for make_empty_image and updates the unit test
Changes fill options for make_empty_image function in gammapy.image.utils
Minor formatting change
Merge pull request #113 from ellisowen/cube_to_image_bug_fix

Fixes bug in cube_to_image function for case where no slicepos given