Random engines & distributions as proper C++11 classes

Random engines & distributions as proper C++11 classes

This change implements the ThreeFry2x64 random engine with flexible
number of encryption rounds and internal counter bits. The class is
compatible with the C++11 random number generators, and the GROMACS
tabulated normal distribution has likewise been turned into a
random distribution compatible with C++11, meaning they can be used in
almost any combination with the standard library distributions.
- The ThreeFry2x64 implementation uses John Salmon's idea of a template-
  selected internal counter so a number of bits are reserved to generate
  an arbitrary random stream. This makes it possible to use ThreeFry as
  a normal random engine, and even in counter mode it is possible to
  draw an arbitrary amount of random numbers before restarting counters.
- Both accurate (20-round) and fast (13-round) versions are available.
- There is a gmx::DefaultRandomEngine when we don't care about details.
- gmx::GammaDistribution has been added to work around bugs in
  libstdc++-4.4.7 headers, and to avoid getting different results
  for libstdc++ vs. libc++.
- Custom Uniform, normal, and exponential distributions have been added
  to make all results reproducible across platforms since stdlibc++ and
  libc++ do not use the same generating algorithms.
- Code using random numbers has been updated, but no changes have been
  made to turn random seeds into 64bits yet.
- The selection nbsearch unit test was a bit fragile and very sensitive
  to the coordinate specific values; this has been fixed so it should
  be resilient no matter what RNG is used in the future.

Change-Id: I47a04d03e2f264e1a6ef0aa0a2174cb464ed9af7