Implements additional unit tests for integral_flux_image in spectral_cube
Fixes mistake in angle units definitin in image utils.py and affected npred_general tutorial script
Merge with master
Updates spectral cube test in line with angle unit corrections
Minor corrections to npred_general.py and test_spectral_cube.py
Update
Merge pull request #188 from ellisowen/issue_178_test

Add more unit tests for spectral_cube.integral_flux_image