BUG: Fixed xrange probem in MixedLM profile; fixed complex number problem in MixedLM Hessian

Small but important bug fixes:

* Error in profile_re, which previously had no test.  I fixed the error and added a test.

* Basic test coverage for EM

* Fix the issue that was producing a warning about complex numbers.

It appears that approx_hess_cs will use complex-valued params when computing numerical derivatives.   If we copy part of params into another structure, it also needs to be capable of holding complex values. I was getting strange behavior before making this change.