SOLVERS: prefer cfdemSolverPimple over cfdemSolverPiso by default

[marniemann]

Summary

• Flips the default solver recommendation: cfdemSolverPimple should be preferred over cfdemSolverPiso even for laminar/single-outer-corrector cases — it runs safely in Piso-equivalent mode (nOuterCorrectors 1) with no performance penalty, is generally more accurate for coupled CFD-DEM, and exposes more features (turbulence, smoothing models, expCorrDeltaUError) if the case grows into needing them.
• cfdemSolverPiso is now documented as the fallback for installs that don't build cfdemSolverPimple, with a check (ls $CFDEM_APP_DIR | grep cfdemSolver / which cfdemSolverPimple) for confirming availability before assuming it's present.

Test plan

• Confirm the updated guidance matches current internal recommendations
• Verify wording doesn't need a similar update anywhere else referencing solver choice