Restructure the ocean package

Summary of changes:

• Remove duplicate cmod5n module
• Remove unused image_processing module
• Create a new scene_preparation module and move model readers and scene_tools to it
• Move spectrum, backscatter, and Doppler computation modules to the forward_models package
• Move wave_spectra_inversion_tools and retrieval_tools to the inversion package
• Move read and write tools to new io package.