Multiple_GGmax_Curves.get_curve_matrix() and Multiple_Damping_Curves.get_curve_matrix() raise ValueError when a shorter curve starts at strain 0 #109

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

Problem

When the curves have different lengths, get_curve_matrix() resamples each shorter curve with np.geomspace(np.min(strain), np.max(strain), max_length). np.geomspace() cannot start at 0, so if a shorter curve has a strain of 0, get_curve_matrix() raises ValueError: Geometric sequence cannot include zero. The curve classes accept a strain of 0 (check_two_column_format(..., ensure_non_negative=True) only rejects negative values), and curves of equal length with a strain of 0 work, because they are not resampled.

import numpy as np

from PySeismoSoil.class_curves import (
    Damping_Curve,
    GGmax_Curve,
    Multiple_Damping_Curves,
    Multiple_GGmax_Curves,
)

long_strain = np.array([0.0001, 0.001, 0.01, 0.1, 1.0])  # 5 points, unit: %
short_strain = np.array([0.0, 0.01, 0.1, 1.0])  # 4 points, starts at 0

GGmax_long = np.column_stack((long_strain, [1.0, 0.98, 0.9, 0.6, 0.2]))
GGmax_short = np.column_stack((short_strain, [1.0, 0.9, 0.6, 0.2]))
damping_long = np.column_stack((long_strain, [1.0, 1.2, 2.0, 6.0, 15.0]))
damping_short = np.column_stack((short_strain, [1.0, 2.0, 6.0, 15.0]))

# Zero strain is accepted by the curve classes
print(GGmax_Curve(GGmax_short).strain, Damping_Curve(damping_short).strain)
# [0.   0.01 0.1  1.  ] [0.   0.01 0.1  1.  ]

# Same lengths: works
mgc = Multiple_GGmax_Curves([GGmax_short, GGmax_short])
print(mgc.get_curve_matrix().shape)
# (4, 8)

# Different lengths, the shorter curve starts at 0
for curves in [
    Multiple_GGmax_Curves([GGmax_long, GGmax_short]),
    Multiple_Damping_Curves([damping_long, damping_short]),
]:
    try:
        print(curves.get_curve_matrix().shape)
    except ValueError as e:
        print(f'{type(e).__name__}: {e}')
# ValueError: Geometric sequence cannot include zero
# ValueError: Geometric sequence cannot include zero
# (expected (5, 8) for both)

Fix

1. In both get_curve_matrix() methods, build the resampled strain array so that it can start at 0, for example:

   if np.min(strain) > 0:
       strain_ = np.geomspace(np.min(strain), np.max(strain), max_length)
   else:
       positive = strain[strain > 0]
       strain_ = np.concatenate(
           ([0.0], np.geomspace(np.min(positive), np.max(strain), max_length - 1))
       )

   For the 4-point curve above this gives [0, 0.01, 0.0464, 0.215, 1.0] (rounded), and np.interp() then gives G/Gmax [1.0, 0.9, 0.779, 0.549, 0.2] (rounded). The same code is in both classes, so it could move into one shared helper.
2. Add a test with two curves of different lengths where the shorter one starts at strain 0, for both Multiple_GGmax_Curves and Multiple_Damping_Curves.