Fix Slab geometry thickness/steepness argument order in MembraneInfo (#719) (#722)
## Summary
Fixes #719. `Slab`'s constructor is `Slab(core::Real steepness,
core::Real thickness)`, but two `MembraneInfo` constructors in
`core/conformation/membrane/MembraneInfo.cc` called it with the
arguments in the wrong order.
**(1) Plain 6-arg constructor** passed `Slab(thickness, steepness)`, so
the `Slab` geometry stored thickness and steepness swapped. The
`fa_2012` energy methods (`FaMPEnv`/`FaMPSolv` →
`mpframework_smooth_fa_2012`) read thickness/steepness off the geometry,
so this silently changed scores (e.g. with `thickness=15.6,
steepness=10`, the IMM1 transition midpoint became 10 Å instead of 15.6
Å with a far steeper transition). Now passes `Slab(steepness,
thickness)`.
**(2) Implicit-lipid constructor** passed `Slab(thickness, steepness)`,
where the bare `thickness` token was *not* the member `thickness_` (just
set from `implicit_lipids_->water_thickness()`) but the unscoped
`MEM::thickness` enum (`== 1`) from `MembraneParams.hh` — both live in
`core::conformation::membrane`. The call was therefore effectively
`Slab(1, steepness)`: the lipid water thickness was dropped and `1`
landed in the steepness slot. Now passes `Slab(steepness, thickness_)`,
matching the sibling lipid + `MP_GEOMETRY_TRANSITION` constructor's
`SLAB` case. This path is currently latent (these constructors set
`implicit_lipids_`, so `use_franklin()` is true and the geometry
thickness/steepness aren't read), but it was still wrong.
All other `Slab` call sites in the file already use the correct
`(steepness, thickness)` / `(steepness, thickness_)` order, so these two
lines are the complete set of violations.
Diagnosis and reproduction by @vuoanh in #719.