added tests of elliptical slice sampler to test-mcmc.R
added tests of elliptical slice sampler to test-mcmc.R

nimble-dev/nimble,nimble-dev/nimble,nimble-dev/nimble,NLMichaud/nimble,nimble-dev/nimble,nxdao2000/nimble,NLMichaud/nimble,nxdao2000/nimble,nxdao2000/nimble,nimble-dev/nimble,NLMichaud/nimble,nxdao2000/nimble,NLMichaud/nimble