Fixing Saturated Pixel Scaling: closes #21

mtpipeline_test.py
    make_desired_result:
        - Fixed because it was using the same code to generate the test and truth data!
    make_output_file_dict_test:
        - Changed the test to be more consise.

mtpipeline.py:
    run_mtpipeline:
        Made a small change so the weight map filename is passed to run_trim.
    make_output_file_dict:
        Added a for loop to generate the weight map FITS filenames.

run_trim.py:
    subarray:
        Added max() functions to ensure the lower bounds of the subarray are >= 0.
    replace_by_weight:
        Created the replace by weight function to replace any pixel with a weighted value of 0 with the local 3x3 median.
    PNGCreator.saturated_clip:
        Updated this to call replace_by_weight and create a stat image.
    get_fits_data:
        Created this small function just cuz.