parameter: replace Gene by its container, Chromosome

  It turns out that having the atomic type of genes wasn't very
  useful. This change's only implication is that the
  PrintableBreeeder does not longer require the Gene to be
  individually printable, but the chromosome.

  * README.md: reflect these changes.
  * breeder.hh, breeder.hxx, breader_thread.hxx: simple
    substitution.
  * main.cc: update the parameter here.
  * printablegene.hh: this is now moot, as the QueenSolver now
    has need for an overload.
  * queensolver.hh: define an operator<<.