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  mcmc_multi is a subclass of the mcmc class to create markov chain
  monte carlo simulations on multiple chains simultaneously.
  One of IDL's principal weaknesses is that, since it is an
  interpreted scripting language, it is much slower than compiled
  languages for repetitive tasks like looping. The mcmc_multi class
  tries to mitigate this problem by vectorizing the mcmc code. In
  this class, multiple chains are run simultaneously and, assuming
  that the code which generates and evaluates trial links is
  vectorized well, this may be substantially faster than running one
  long mcmc chain.
  To use this class, the user must supply ARRAYS for the seed and
  data variables when initializing the object. Each element in the
  array represents the seed/data for a particular chain. The final
  set of chains fetched from the getChain method will have nchain
  columns and nstep rows.
  As with the mcmc class, the user must also override the
  logTargetDistribution and selectTrial methods in a subclass of
  mcmc_multi. These methods have the same signatures as in a normal
  mcmc object, but the inputs and outputs must now be arrays, not
  scalars. These objects should be written in a vectorized fashion,
  otherwise mcmc_multi will probably have no speed bost over mcmc.
  Some quick testing showed that mcmc_multi was about a factor of 2
  slower than mcmc when run using only 1 chain, but was 10x faster
  when comparing 100 chains to 1 chain with 100x as many steps.

Modification History

  October 2009: Written by Chris Beaumont, adapted from the mcmc
  December 2009: Added tracking of maximum posterior

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