Multiple calls to ts::rnd::dugauss() resulted in identical sequences of random
numbers.

Issue regarding random number generator in libtsxx
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Location: src/libs/libtsxx/random.cc

The function ts::rnd::dugauss() allocates a GSL random number generator and
immediately seeds it. The seeding operation is required, since the default
seed is a constant, which would result in identical sequences. Function
ts::rnd::dugauss() uses the current system time as returned by function time()
as a seed value. Unfortunately, if ts::rnd::dugauss() is called several times
during the same second, this also will result in identical sequences.

Currently ts::rnd::dugauss() is only used by src/ts/wf/noisymize.cc and by the
filter GaussianNoise in libtsxx.

The appropriate solution appears to be:
- encapsulate the GSL randon number generator in a class object, just like is
  provided in tfxx::numeric::RNGgaussian
- use a static variable of this class type in ts::rnd::dugauss()
  this variable will be initialized only once upon the first call to the
  function
- the destructor is guaranteed to be called after termination of the program,
  such that the destructor is able to properly call gsl_rng_free