Fix NameError in Subsystem.getYawStiffness (undefined 'l')

[gaoflow]

Calling Subsystem.getYawStiffness() always raises NameError: name 'l' is not defined.

The bug

def getYawStiffness(self):
    tau0 = -self.fB[0]  # horizontal tension component [N]
    yaw_stiff = (tau0/l)*self.rad_fair**2 + tau0*self.rad_fair  # [N-m]   # <- l undefined
    return yaw_stiff

l is never assigned in the method (no local, no attribute, no global), so every call fails. git blame points to commit 2f330c8 "Subsystem updates for span consistency" — the rename of the line length to span missed this one site, leaving the stale l.

The fix

Use self.span (the horizontal end-to-end distance, set in __init__), which is the length the formula intends:

yaw_stiff = (tau0/self.span)*self.rad_fair**2 + tau0*self.rad_fair  # [N-m]

Dimensionally consistent: [N]/[m]·[m²] + [N]·[m] = [N·m].

Reproduction (before this PR)

import numpy as np
import moorpy as mp

ss = mp.Subsystem(depth=100, span=200, rad_fair=10, z_fair=-10)
ss.fB = np.array([-1.0e5, 0.0, 5.0e4])   # horizontal fairlead force
ss.getYawStiffness()        # -> NameError: name 'l' is not defined

Verification

• New regression test tests/test_subsystem.py::test_getYawStiffness_runs asserts the call returns the documented value (tau0/span)*rad_fair**2 + tau0*rad_fair. It fails on dev (NameError) and passes with this fix.
• Full test suite: 29 passed.

[shousner]

Note to self that Subsystem.getYawStiffness() is not used anywhere within MoorPy or FAD-Toolset (but LineDesign does have its own getYawStiffness method, which is not exactly the same)