Save the internal state and restart about trng4

[rabauke]

Dear @dessteph, there are two ways to save the internal state of an engine:

1. Copy an instance to another one.
2. Write an engine into an i/o stream and read this output later again.

The following code snipped illustrates approach 1.

#include <cstdlib>
#include <iostream>
// include TRNG header files
#include <trng/yarn2.hpp>
#include <trng/normal_dist.hpp>

int main() {
  // random number engine
  trng::yarn2 R;
  // normal distribution with mean 6 and standard deviation 2
  trng::normal_dist<> normal(6.0, 2.0);
  trng::yarn2 R_backup;
  // save the internal state of R into R_backup
  R_backup = R;
  // generate 10 normal distributed random numbers
  for (int i{0}; i < 10; ++i)
    std::cout << normal(R) << '\n';
  // restore internal state from R_backup
  R = R_backup;
  // generate the sam 10 normal distributed random numbers again
  for (int i{0}; i < 10; ++i)
    std::cout << normal(R) << '\n';
  return EXIT_SUCCESS;
}