Tempus: Add accessor to WrapperModelEvaluator

Created by: ccober6

For a segregated linear solve, EMPIRE needs access to the WrapperModelEvaluator to get to alpha and beta. Adding simple accessors to it. All tests pass.

#4970 (closed)

@trilinos/tempus

Checklist

• My commit messages mention the appropriate GitHub issue numbers.
• All existing tests passed.
• No new compiler warnings were introduced.