change local path to images to url in Introduction Notebook

[fedhere]

local paths break notebooks if single notebooks are downloaded, e.g. Introduction Notebook has a path to an image for comparison with the results produced locally. If it is just a path to an image that needs to be displayed, but not to something functionally important to the notebook that should be downloaded, it should be replaced with URL for people that download a single notebook instead of cloning the repo

Image(filename='images/thumb.opsim_Mean_fiveSigmaDepth_i_HEAL_SkyMap.png')

could be replaced by

Image(url='https://raw.githubusercontent.com/LSST-nonproject/sims_maf_contrib/master/tutorials/images/thumb.opsim_Mean_fiveSigmaDepth_i_HEAL_SkyMap.png').