Tempus: Separate Stepper Header and Source

Created by: ccober6

Cleanup the Stepper base class and separate the header from the source code. All tests pass on a Mac.

#4843 (closed)

@trilinos/tempus

Checklist

• My commit messages mention the appropriate GitHub issue numbers.
• My code follows the code style of the affected package(s).
• All new and existing tests passed.
• No new compiler warnings were introduced.