Provide email address to NEOS solvers

[jsiirola]

Fixes #N/A

Summary/Motivation:

NEOS has updated its requirements and not rejects any jobs that do not provide an email address as part of the XML problem definition. This PR adds an the <email> field to the document specification. The interface uses the contents of the EMAIL environment variable (if present) otherwise reverts to user@{fully-qualified hostname}. If neither of those appear to be a valid email address, then the Kestrel interface raises an RuntimeError

Changes proposed in this PR:

• add <email> section to the NEOS XML document
• switch the username to be the email address (instead of {user} on {host}). This may help address NEOS interface broken for certain solvers  #910.
• retrieve the email address from the EMAIL environment variable, or use user@{fully-qualified hostname}

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