Enable subcycle time stepping in CarpetX schedule

[lwJi]

Summary Refactored driver.cxx and schedule.cxx to enable subcycling support in CarpetX.

Change Log

1. Regression Test: Added a WaveToyX test (3 refinement levels) to ensure backward compatibility.

2. Parameter Scope: Promoted use_subcycling_wip to RESTRICTED scope for external thorn usage.

3. Driver Logic: Enabled subcycling during regridding (provided levels align) in driver.cxx.

4. Schedule Logic: Updated schedule.cxx to support the new subcycling flow.

5. Unit Test: Added a new test case specifically for subcycle time stepping.

[eschnett]

I made a few suggestions. Please look at them, then merge.