metropolis_chain: Do a metropolis chain
In jmzobitz/SoilModeling:

metropolis_chain Runs a MCMC metropolis chain

1 2	metropolis_chain(param_in, data_in, model, join_key, iterations, tuning, estimate_chain, mcmc_parameters)

`param_in`	input parameter vector - is a data frame with the following columns: 1. name 2. changeable 3. value 4. minVal 5. knob 6. sampled
`data_in`	input data we use to compare in the likelihood function.
`model`	the particular model we are using in the mcmc
`join_key`	The name of the variable we will join to data_in when doing likelihood
`iterations`	The number of times we want to iterate through this chain
`tuning`	a T/F value depending on if we want to limit the searchable range of parameters
`estimate_chain`	a T/F value depending on what we want to return.
`mcmc_parameters`	a data frame of how we want to estimate (can be mcmc_superfast or mcmc_defaults)