Initial reorganization of code

[JuanCab]

As part of the refactoring, I have performed an initial code reorganization:

• I moved functions in visualization to gui_tools and plotting
• I moved some functions from other directories to utils
  • Moved utility functions for stellar comparisons: read_file, set_up, crossmatch_APASS2VSX, mag_scale, and in_field moved from gui_tools/comparison_function.py to utils/comparison_utils.py
  • Moved utility functions for catalog searches: catalog_search.py from differential_photometry to utils
  • Moved utility functions for magnitude transforms: magnitude_transforms.py from differential_photometry to utils

In the process of performing these moves, I searched for each call to each function in the source code and fixed all imports to work. I also moved some of the testing files around. I did confirm pytest runs to completion.

Miscellania: I also replaced several import pandas with import pandas as pd and fixed the corresponding function calls as this is fairly standard. I also fixed a deprecated call to the astropy function scale_image which was removed in astropy 2.0!