Fix parallel timer classes.
Fix parallel timer classes.

- use references in range-based for loop (fixes bug in ChTimerParallel::Reset)
- pass timer names as const references
- use std::map::at instead of operator[] to prevent insertions if a key does not already exist
- fix bug in ChTimerParallel::PrintReport

projectchrono/chrono,Milad-Rakhsha/chrono,armanpazouki/chrono,projectchrono/chrono,projectchrono/chrono,armanpazouki/chrono,Milad-Rakhsha/chrono,rserban/chrono,Milad-Rakhsha/chrono,armanpazouki/chrono,armanpazouki/chrono,rserban/chrono,projectchrono/chrono,dariomangoni/chrono,rserban/chrono,projectchrono/chrono,dariomangoni/chrono,dariomangoni/chrono,armanpazouki/chrono,dariomangoni/chrono,Milad-Rakhsha/chrono,Milad-Rakhsha/chrono,rserban/chrono,dariomangoni/chrono,projectchrono/chrono,dariomangoni/chrono,Milad-Rakhsha/chrono,rserban/chrono,armanpazouki/chrono,rserban/chrono,rserban/chrono