fix broken tests:

- mb_ferromagnet_diff_repro
- mb_ferromagnet_diff_minimal_repro

as the comment hinted:
> these tests probably failed earlier because they were allowing negative mu_r values.
> no (ordinary?) material has a negative permeability.
> most materials (except superconductors) have >= 1.0 relative permeability
> permeability = mu = B/H (or, \Delta B/\Delta H)

in fact, the relative permeability was -0.56.
it's now 1.39

crates/cross/src/mat/mb_pgram.rs

@@ -33,8 +33,9 @@ pub struct MBPgram<R> {
     pub max_m: R,
 }
 
-/// f(x0) = -ymax
-/// f(x1) = ymax
+/// evaluate `f(x)` at the provided `x`, where `f` is defined by these constraints:
+/// 1. f(x0) = -ymax
+/// 2. f(x1) = ymax
 fn eval_bounded_line<R: Real>(x: R, x0: R, x1: R, ymax: R) -> R {
     let x = x - x0;
     let x1 = x1 - x0;