process types registry

[eagmon]

This is another possible fix to the cores everywhere. I made a process_type_registry which includes a ProcessType called 'core', which is the default core that Composites and Processes can use. People can add to this core if they want to, or make new cores and then just pass a string into Composite to look up the appropriate core.

I think there may need to be something smarter with how Processes are assigned cores, without having our users needing to thread cores through everywhere -- I don't want Processes to have to do the whole core=core through the super. Can Composite ensure each Process is given the correct core by setting it rather than having to weave it in through the init?