Wsm6 microphysics initial Fortran setup

[jmsexton03]

This PR adds initial WSM6 support in ERF, using physics_mmm as the basis for the WSM6 implementation and interface wiring.

What this PR adds

• New WSM6 integration under Source/Microphysics/WSM6/ (Fortran bridge + ERF init/advance/update plumbing).
• Build-system wiring for WSM6 sources and interfaces.
• Moisture/microphysics path updates needed to run WSM6 through ERF’s existing framework.

Implementation note

WSM6 is integrated as a separate path from Morrison and is based on the physics_mmm WSM6 formulation.
Both models evolve the same six moisture species fields in ERF (qv, qc, qi, qr, qs, qg), but WSM6 is single-moment while Morrison is two-moment.
As a result, number-concentration treatment differs (for example, cloud droplet number handling such as nc_h is not evolved in WSM6 the same way it is in Morrison).

Validation status

• 120-step SquallLine behavior is similar to Morrison.
• Physics are meaningfully different at 3000 seconds.