Determine if the object is a valid MCMC
See also
Use create_mcmc
to create an MCMC
Examples
if (is_on_ci()) {
check_empty_beautier_folder()
# Returns TRUE
is_mcmc(create_mcmc())
is_mcmc(create_ns_mcmc())
# Returns FALSE
is_mcmc("nonsense")
is_mcmc(NULL)
is_mcmc(NA)
is_mcmc("")
is_mcmc(c())
check_empty_beautier_folder()
}