Create a function for getting and plotting DECaLS imaging

[hzovaro]

a la plot_sdss_image. This would be v useful in cases where the galaxy lies outside the SDSS footprint.

• [x] Consider combining sdss.py and decals.py into a single file called photometry.py or something similar.
• [x] Consider making functions that take the photometry source as an input argument & save/load appropriately.
• [x] Instead of having sdss_im_path defined in the top-level of config.json, save the figures into settings.cutout_path.
• [x] ~Use astropy.visualization.wcsaxes.SphericalCircle rather than Circle to plot the FoV of the instrument (see here for details)~ Scrapping this due to a low-level error that's probably due to a package version issue.
• [x] Fix up test .json file & directories

[hzovaro]

There seems to be a strange issue whereby sometimes the path specified in the config file isn't being updated, so that download_cutout_image() tries to save it into the default directory which is cutouts/. No idea how to fix this.