Implement Basic Parallel Capabilities

[mandli]

Propose to add a simple process pool so that multiple workers can take frames until plotting is done rather than use a single process.

Add code into the plotdata object to enable parallel running and parallelize the
png file creation by launching multiple processes that will take jobs as they
finish with each frame.

• Not sure if this will really help as plotting may be a memory problem rather than a compute problem
• This is probably not as efficient as it could be but something better would probably require a lot more refactoring
• Future design should encapsulate better a single frame (or figure's) plotting process to break this down into even more work units
• Multiprocessing has issues with pickle due to simple lambda implementations
• May be able to provide initialization but this would require a refactor of plotpages.plotclaw_driver

[mandli]

Basic functionality added in #174.