Making the library to work without PAPI

so that a code can still use the same interface to get the times

Added two files to isolate the ostreams to print results. Before they were in Papi class, but I think it would make sense to move them somewhere else.

The code uses a compile time flag NOPAPI and you should compile PapiCounter_nopapi.cpp instead of PapiCounter.h. I guess an alternative approach is to #ifdef the whole implementation in PapiCounter.h and the compilation process would depend on the NOPAPI flag only. Up to you.