Unqualified abs() causes NaNs and missing dispersion roots with GCC 15

[YuanYusung]

Problem

On macOS Apple Silicon with GCC 15.2, forward returns no Rayleigh-wave dispersion points for a valid layered model.

Dense secular-function scans show that most values become NaN.

Cause

Several floating-point expressions in src/secfunc.cc use unqualified abs():

wvnom = abs(wvno - xkb);

In this build, the integral overload is selected. For example, a difference of about 0.998 becomes 0, causing rb = 0 and eventually 0 / 0 -> NaN.

src/disp.cc contains a similar call in the adaptive sampling logic.

Suggested fix

Include <cmath> and use std::abs() consistently:

#include <cmath>

wvnom = std::abs(wvno - xkb);

Environment: macOS Apple Silicon, GCC 15.2.0, Eigen 3.4.1.