plot_dependency_graph: reduced resulting image size, made cooling ghosts blue

The last update of the task_plot_dependencies.py made the resulting image a bit too big for comfort, as in my default image viewer started lagging when displaying it. So I reduced the resulting image size, trying to keep everything else pretty much the same.

Also, I made the cooling ghost tasks have the same colour as the other hydro type tasks.

For comparison, this is what the script produces before this merge request:

And this is the new version.