_calc_vertical_stress() changes the caller's thickness when the last layer is 0 m #120

Found while reviewing #74. Pre-existing on main (b764057); #74 does not change this code.

Problem

_calc_vertical_stress() writes into the caller's thickness array when the last thickness is 0. It sets that entry to 1 m so the halfspace still contributes to the overburden. The arrays passed in by hh_param_from_profile(), hh_param_from_curves(), and Damping_Calibration.get_damping_curves() are views (vs_profile[:-1, 0]), so the write would change the profile.

Those callers drop the real halfspace first, and check_Vs_profile_format() rejects a zero thickness above the last layer, so a valid profile does not hit this today. Calling the helper with a trailing 0 does:

import numpy as np
import PySeismoSoil.helper_hh_calibration as hhc

h = np.array([5.0, 0.0])
rho = np.array([1800.0, 2000.0])
hhc._calc_vertical_stress(h, rho)
print(h)  # [5. 1.]  -- the 0 m halfspace was changed to 1 m

The same function's density warning names __calc_vertical_stress (two underscores). The function is _calc_vertical_stress.

Fix

Copy h before replacing a zero last thickness, or use a local thickness of 1 m in the stress formula without assigning into h. Print _calc_vertical_stress in the warning.