Fix add_nodes for conical shaft elements

[jguarato]

Resolves #1218

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This PR fixes incorrect behavior in Rotor.add_nodes when applied to conical (tapered) shaft elements. The method now correctly interpolates diameters and lengths when splitting elements, preserving geometry and connectivity.

Notes

• Updated add_nodes to handle conical elements.
• Correct interpolation of inner/outer diameters and element lengths.
• Fixed geometry issues when inserting nodes into conical shaft elements and ensuring rotor models remain consistent.

Example

import numpy as np
import ross as rs
from ross.materials import steel

L = 0.20

shaft_elements = [
    rs.ShaftElement(L=L, idl=0.0, odl=0.04, material=steel),
    rs.ShaftElement(L=L, idl=0.0, odl=0.04, material=steel),
    rs.ShaftElement(L=L, idl=0.0, odl=0.04, idr=0.0, odr=0.07, material=steel),
    rs.ShaftElement(L=L, idl=0.0, odl=0.07, material=steel),
    rs.ShaftElement(L=L, idl=0.0, odl=0.07, material=steel),
]

disk = rs.DiskElement(n=2, m=5.0, Ip=0.01, Id=0.02)
bearing0 = rs.BearingElement(n=0, kxx=1e6, kyy=1e6, cxx=100, frequency=[0])
bearing1 = rs.BearingElement(n=5, kxx=1e6, kyy=1e6, cxx=100, frequency=[0])

rotor = rs.Rotor(shaft_elements=shaft_elements,
                 disk_elements=[disk],
                 bearing_elements=[bearing0, bearing1],
                 tag="Rotor_with_conical_element")

new_rotor = rotor.add_nodes([0.5, 0.3, 0.66])

rotor.plot_rotor().show()

new_rotor.plot_rotor().show()

Before:

Now fixed: