Fixing PressurelessCollapse test problem. A combination of a small Courant safety factor and relatively large tiny_number led to undesired results that stop the simulation too soon. This change increases the safety factor, and returns tiny_number to the default of 1.0e-20. Also cleaned up the parameter file to more accurately reflect the intended behavior.
Added a make_plots.py, which can be used to make a quick look at the resulting simulation.