Prepare for upcoming SOILWAT2 release v8.0.0

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This PR updates the STEPWAT2 repository to work with upcoming SOILWAT2 v8.0.0 such that STEPWAT2 behavior and output remain unchanged.

However, STEPWAT2 could consider utilizing some of the new SOILWAT2 functionality (which this PR does not implement):

• Use the new SOILWAT2 internal spin-up functionality (which is deactivated by default).
• Future development of gridded STEPWAT2 could build on the new domain functionality of SOILWAT2 (and, potentially, input/output via netCDFs).

Details about the SOILWAT2 release v8.0.0:

• PR: https://github.com/DrylandEcology/SOILWAT2/pull/378
• Milestone: https://github.com/DrylandEcology/SOILWAT2/milestone/16