Consider using `subtree` instead of `submodule` for `SOILWAT2`

[dschlaep]

Issues with SOILWAT2 as submodule

• #175
• packages that have rSOILWAT2 as dependency cannot utilize github actions with recent versions of r-lib/actions that use r-lib/actions/setup-r-dependencies (v2 or v1) to install R package dependencies. The action setup-r-dependencies uses the R package pak instead of remotes. pak does not support submodules and thus fails to install rSOILWAT2 as a dependency, e.g.,
  • DrylandEcology/rSW2funs#4 (comment)
  • with a potential workaround as here: DrylandEcology/rSW2funs@6744004