Note on draws_out in mcmc::hmc()

[Tadinu]

Hi @kthohr,

Thanks alot for the very cool repo!
May I just point out some notes, and would so appreciate if you could help clarify.

Using mcmc as a shared library,
(1) In hmc_normal_autodiff example, when I turn main() into a class member function, it was observed that draws_out needs to be static or class member, otherwise a double free or corruption error is thrown after mcmc::hmc() is called.

(2) Also in, may you tell why the result of autodiff::gradient() can't be assigned directly to *grad_out?
Thank you!