Make Parallel into a module, not a class

[jhtu]

I see no reason for Parallel to be a class. In fact, we have to do funny stuff to ensure or encourage the user not to have multiple instances of it. It seems to me simpler if we just make it a module that has a number of useful methods. This avoids the whole instance issue altogether, and should provide the same functionality.

[jhtu]

Changed parallel into a module only, not a class.