Integer & time measurement types made uniform

[haykh]

• All integer types related to particle counting (ranges, numbers, strides), now have npart_t type (aliased to unsigned long long int);
• All integer types related to the grid resolution (# of cells etc.), now have ncell_t type (aliased to std::size_t);
• Integer timestep & timestep intervals now have timestep_t (aliased to std::size_t);
• Code-time in simulation units as well as time intervals have simtime_t (aliased to double);
• Real-world time duration (e.g., for timers) has a type of duration_t (aliased to double).

This allows to easily & consistently change these types throughout the code (as per weird behavior found on Frontier with long double and std::size_t).