Problem with insert_knot function when splitting surface #179

Describe the bug
When I call the "split_surface_u" function with the value of 0.1 for the param, I get a ValueError exception about the knot vector for the u-direction.

To Reproduce
Steps to reproduce the behavior:

1. Here is the sample code to reproduce the error.

from geomdl import NURBS
from geomdl import operations
import matplotlib.pyplot as plt

surf = NURBS.Surface()
surf.degree_u = 4
surf.degree_v = 4
surf.set_ctrlpts([[0.0, 0.0, 0.0, 1], [2.0, 0.0, 0.0, 1], [4.0, 0.0, 0.0, 1], [6.0, 0.0, 0.0, 1], [8.0, 0.0, 0.0, 1], [10.0, 0.0, 0.0, 1],
[0.0, 2.0, 0.0, 1], [2.0, 2.0, -1.0, 1], [4.0, 2.0, -1.0, 1], [6.0, 2.0, -1.0, 1], [8.0, 2.0, -1.0, 1], [10.0, 2.0, 0.0, 1],
[0.0, 4.0, 0.0, 1], [2.0, 4.0, -1.0, 1], [4.0, 4.0, 1.0, 1], [6.0, 4.0, 1.0, 1], [8.0, 4.0, -1.0, 1], [10.0, 4.0, 0.0, 1],
[0.0, 6.0, 0.0, 1], [2.0, 6.0, -1.0, 1], [4.0, 6.0, 1.0, 1], [6.0, 6.0, 1.0, 1], [8.0, 6.0, -1.0, 1], [10.0, 6.0, 0.0, 1],
[0.0, 8.0, 0.0, 1], [2.0, 8.0, -1.0, 1], [4.0, 8.0, -1.0, 1], [6.0, 8.0, -1.0, 1], [8.0, 8.0, -1.0, 1], [10.0, 8.0, 0.0, 1],
[0.0, 10.0, 0.0, 1], [2.0, 10.0, 0.0, 1], [4.0, 10.0, 0.0, 1], [6.0, 10.0, 0.0, 1], [8.0, 10.0, 0.0, 1], [10.0, 10.0, 0.0, 1]], 6, 6)
surf.knotvector_u = [0.0, 0.0, 0.0, 0.0, 1.0 / 4.0, 1.0 / 2.0, 3.0 / 4.0, 1.0, 1.0, 1.0, 1.0]
surf.knotvector_v = [0.0, 0.0, 0.0, 0.0, 1.0 / 4.0, 1.0 / 2.0, 3.0 / 4.0, 1.0, 1.0, 1.0, 1.0]

s1, s2 = operations.split_surface_u(surf, 0.1)

eps = 1.0e-6

#Plot the NURBS surface
x = []
y = []
z = []

du = 0.025
dv = 0.025
start_u = 0
start_v = 0
finish_u = 1
finish_v = 1

u = start_u

while (u < finish_u + eps):
v = start_v

while (v < finish_v + eps):
point = surf.evaluate_single((max(0, min(u, 1)), max(0, min(v, 1))))
x.append(point[0])
y.append(point[1])
z.append(point[2])
v += dv

u += du

fig = plt.figure()
ax = fig.add_subplot(projection='3d')
ax.set_aspect('equal')
ax.scatter(x, y, z, marker="o")

2. The error output should be:
   File "d:\tmp\test_nurbs_surface.py", line 19, in
   s1, s2 = operations.split_surface_u(surf, 0.1)
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "D:\Programmes\Python\Python312\Lib\site-packages\geomdl\operations.py", line 1115, in split_surface_u
   insert_knot_func(temp_obj, [param, None], num=[r, 0], check_num=False)
   File "D:\Programmes\Python\Python312\Lib\site-packages\geomdl\operations.py", line 126, in insert_knot
   obj.knotvector_u = kv_u
   ^^^^^^^^^^^^^^^^
   File "D:\Programmes\Python\Python312\Lib\site-packages\geomdl\abstract.py", line 1375, in knotvector_u
   raise ValueError("Input is not a valid knot vector for the u-direction")
   ValueError: Input is not a valid knot vector for the u-direction

Expected Behavior
What I expected to happen is no bug and two splitted surfaces.

Configuration:

• OS: Windows 10
• Python distribution: python.org
• Python version: 3.12.4
• geomdl install source: PyPI
• geomdl version: 5.3.1

Screenshots
Here is the surface that I want to split at u=0.1.