Get / set the email address NEOS requires for job submission. set_neos_email()
stores it in the energyRt option and exports it to the NEOS_EMAIL
environment variable, so it is picked up by BOTH backends: the GAMS/NEOS
backend (R-side, neos_submit_job()) and the Pyomo/NEOS backend (which reads
NEOS_EMAIL from inside the python subprocess). get_neos_email() returns the
option, falling back to the NEOS_EMAIL environment variable, or NULL.
