Minor updates and bug fixes

o Fix g++ -std=gnu++14

o oneStepPredict: Add one-step mean to output for methods
'oneStepGaussianOffMode' and 'oneStepGeneric'.

o Fix warnings (windows)

o SUPPORT_OPENMP deprecated - see 'Writing R Extensions'

o Array class changes:

• More explicit array assignment from other class:
  Can now assign from matrix and un-evaluated expression template
  E.g. a = a.matrix() * a.matrix();
• Documentation: warning about the 2D case
• Re-implement vec() operator (avoid loop)
• Add method matrix() to array class:
  Keeps first dimension and collapses remaining dimensions.

o Clean up potential naming conflicts caused by Rmath macros

o atomic_macro.hpp: clang-format-3.6 -i -style='Google'

o Atomic functions general speedup:

• Reduce dynamic allocation overhead caused by creation of new CppAD::vectors.
• Avoid copy overhead betweeen 'vector' and 'CppAD::vector'.
• Managed by som new typedefs.

o bugfix: precompile broke REGISTER_ATOMIC

o Update README.md

o NAMESPACE add stats::median (used by mcmc)