Add initial_image input type

[rmjarvis]

This PR adds the ability for the config processing to read in an initial image from a file and draw additional things onto that. The implementation is pretty simple:

input:
    initial_image:
        file_name: inital_image_file.fits

My test for this is to simulate a supernova time series. It draws a single reference image, and then draws a point source with variable flux 12 times. The SN appears between the 3rd and 4th image. And then it decays exponentially.