Implements the Unadjusted Langevin Algorithm (ula()) and the
Metropolis-Adjusted Langevin Algorithm (mala()) with a C++ backend
using Rcpp and RcppArmadillo.
Provides an S3 class "langevin_chain" with print(), summary(),
and plot() methods.
All random number generation is routed through R's RNG state, so
set.seed() controls reproducibility of the chains.