Reduce memory usage when generating morph plots and a few minor fixes

[sanjayankur31]

• pop dict items as they are consumed to limit memory requirements
• fix minor bug in plot title generation
• quietly skip included files if they cannot be found during plot generation from Cell/NeuroMLDocument objects; when a file is passed, we are able to figure out where the includes are relative to the file, but not when the user passes a Cell/NeuroMLDocument object, where we assume they've loaded things correctly