Adds initial predicted counts cube script
Adds unit test outline (incomplete)
Adds demonstration script to gammapy-examples
update
Fixes compatability issue with gammapy-datasets
update
Add energy_bin_centers_log_spacing utility function
Update npred computation and example with Ellis's latest version
Implement spectral cube coordinate and solid angle image methods
Continue implementation of spectral cube npred computation
Merge pull request #150 from cdeil/npred_cube_fixes

Npred cube fixes