produce_Darendeli_curves() raises TypeError when called with its default strain_in_pct=None #87

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

Problem

produce_Darendeli_curves() documents strain_in_pct=None as "use numpy.geomspace(1e-4, 10, 400)". It does build that array as gamma, but later it takes len(strain_in_pct) instead of len(gamma). With the default, that is len(None), which raises TypeError. The two callers in the package (produce_HH_G_param() and Damping_Calibration.get_damping_curves()) and the tests always pass strain_in_pct, so the default is never run.

import numpy as np

import PySeismoSoil.helper_hh_calibration as hhc

sigma_v0 = np.array([3000.0, 6000.0, 9000.0])

GGmax, xi, gamma_r = hhc.produce_Darendeli_curves(
    sigma_v0, strain_in_pct=np.geomspace(1e-4, 10, 400)
)
print(GGmax.shape)
# (400, 3)

hhc.produce_Darendeli_curves(sigma_v0)  # default: strain_in_pct=None
# TypeError: object of type 'NoneType' has no len()
# (expected the same result as the call above)

Fix

1. Change line 1025 to n_strain_pts = len(gamma). With that change, produce_Darendeli_curves(sigma_v0) returns the same three arrays as produce_Darendeli_curves(sigma_v0, strain_in_pct=np.geomspace(1e-4, 10, 400)).
2. Add a test that calls produce_Darendeli_curves() without strain_in_pct and compares it with the explicit call.