Minor corrections and cleanup of MCEM (#1601)

* Prevent warning of unsampled nodes in buildMCEM.

* Require mcmcse at MCEM build stage.

* Update handling of thinning and appending of new MCMC samples during ascent

* update test-mcem and generate new mcemTestLog_Correct.Rout gold file

* Tweak MCEM non-convergence message.

* Update mcem gold file with slightly different numerical values.

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Co-authored-by: Christopher Paciorek <paciorek@stat.berkeley.edu>

Author: perrydv