metropolis: Create a metropolis hastings updater function.
In grady/evol-mc: Evolutionary MC Project

This function creates and returns a function which implements a Metropolis-Hastings updater.

1	metropolis(target, rprop, dprop = NULL)

`target`	target distribution function. Function should accept current (population of) state(s) as its first parameter and return a vector containing the density evaluation(s).
`rprop`	either a function, or a list of functions. Proposal generator functio(n).
`dprop`	either a function, a list of functions or NULL. Proposal density function(s), NULL value means the proposal density is symmetric.