message: make the default value the same as STAN_NUM_THREADS
diff: #define CMDSTAN_ARGUMENTS_ARG_NUM_THREADS_HPP
  #include <cmdstan/arguments/singleton_argument.hpp>
  +#include <stan/math/prim/core/init_threadpool_tbb.hpp>
  namespace cmdstan {
  @@ -17,8 +18,8 @@ class arg_num_threads : public int_argument {
  #else
  _validity = "num_threads == 1";
  #endif
  - _default = "1";
  - _default_value = 1;
  + _default = boost::lexical_cast<std::string>(stan::math::internal::get_num_threads());
  + _default_value = stan::math::internal::get_num_threads();
  _good_value = 1.0;
  _bad_value = -2.0;
  _constrained = true;
  