CEMMixtureModel removing log_sum_exp redudancy

This is my patch for addressing the issue #2559 
I did this because by doing this, I would not have change the method call from where it has been called.
Ideally, this method shouldn't be present in EM interface, but the patch serves the purpose of reusing the code addressing the issue.
Should I stick with this, or completely remove the method from the EM interface ?