benchmark sample implemented
more tricks for measurements

- better enforce() that now actually does nothing (even no call)
- more units in human() (femtoseconds, attoseconds) and output for
  situation where no appropriate ration units can be found
- zero_line for measurements to weight out cycle from timings
- CPU clock ticks alignment to avoid extra tick in situation when we
  start measure before end of previous tick
- fix threshold to follow original criterion (Haskell) implementation
- basic analyse of samples based on median
enhance zero_line measurement and human()

- use minimum CPU time for zero_line
- remove zero cut-off in measure::run() by relying on more precise
  zero_line
- use duration<double> in human() to get fractions
measurement: better progress
basic bootstrapping
analyse: mean