HPOP plugin engine interface for user-defined equations of motion.

Calc	Calculate method for plugin
Free	Triggered just before the plugin is freed from use to allow for any additional cleanup.
Init	Triggered when the plugin is initialized to allow for any additional needed initialization. Must return true to turn on use of plugin.
Register	Method to register the plugin's inputs, outputs, and events
SetIndices	Gives the plugin the indices into the state vector for its inputs/outputs

Name	Triggered after initialization to set the name of the plugin used in messages.