Refactor spherex_cutouts and spherex_psf #150

#147 transferred the spherex_cutouts and spherex_psf notebooks from the ipac-sp-notebooks repo. @bsipocz suggested several content changes which were not included in that PR. They include:

• Use astroquery.ipac.irsa.Irsa.query_tap
• Use SkyCoord
• When writing the output file, consider setting overwite=False
• Use mpl plotting of astropy quantities
• One of the tip boxes is repeated. Consider keeping just one of them.