Add stub M1 initial guess

[pajkosmi]

Proposed changes

Adds framework to modify the initial guess for the implicit step of the M1 system.

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

Currently, only the explicit solution is used for the initial guess. This file is unused for now.

[pajkosmi]

Yes, the updated M1 system would not yet use this. In fact, the future PRs I have for the M1Grey system makes use of imex::GuessExplicitResult. The purpose of this PR is to have the infrastructure available to change, if we need more accurate guesses. Is it better to just keep these changes on a local branch if that need ever arises and ignore this PR?

[wthrowe]

Yes, let's not add code unless we intend to use it.