Make regular histentry step-based

[otcathatsya]

Right now the regular histentry operates on double, which is only needed for precise spike times. Since the majority of simulations use on-grid simulation steps, it may make sense to change the default to size_t and separate the precise times implementation (potentially through the Time class?).

nest-simulator/nestkernel/histentry.h

Line 40 in b32a3a3

double t_; //!< point in time when spike occurred (in ms)

This would also simplify optimizations relying on grid-locked time steps, e.g. #3247.

[clinssen]

See also #2035