Common hpm library rules

The following rules are common to the hpm library APIs:

• The hpmInit() or f_hpminit() function must be called before any other function in the API.
• The initialization function can only be called once in an application.
• Performance Monitor contexts, like the event set, event group, or counter/event pairs, cannot be reprogrammed at any time.
• All functions of the API are specified as void and return no value or status.