Updated counter-related classes

- Replaced array duplication in Counters (.h/.cpp) by a simple pointer
of pointer (array of Counter* is allocated in main.cpp and used as a
reference in Counters)
- Added a "reset" method in Counters and Counter, allowing to reset
counter values to 0
- Simplified counter incrementation code in main.cpp
- Replaced 'by counters" frame and reset counter access in main.cpp by
direct counter variable access