Source receiver maps do not plot without valid depth and magnitude

[bch0w]

The source receiver map function uses ObsPy Catalog.plot() function to plot the event. However if the Event object does not have a valid depth and magnitude, the plot function fails. However, as mentioned in #71, these values are sometimes unknown and are anyways not critical for the PySEP data gathering routines.

To skirt around this, we can replace the Catalog plot function with a simple ax.scatter() for the event, since its just a single point. Two things that would need to be determined on the fly are 1. where is it plotted (by converting the geographic coordinate into the Cartopy domain), and 2. how large the scatter point should be.