Degree elevation modifies NURBS curve shape

[lslore]

Describe the bug Degree elevation operation via operations.degree_operations modifies the shape of the NURBS curve for particular input curves.

The problem seems to arise when calling split_curve and link_curves in operations.degree_operations. The target NURBS curve is a near 90deg arc and the splitting operation returns the arc itself and an infinitesimal segment, which generates strange behaviour when linked.

To Reproduce Following code reproduces the behaviour:

from geomdl import NURBS, operations
from geomdl.visualization import VisMPL

# Define curve
curve = NURBS.Curve()
curve.degree = 2
curve.ctrlpts = [[821.51962938, -677.395551576, 3.5410634089],
 [873.5806602364927, -682.3571637856359, 88.77621736805291],
 [879.1302220964927, -582.5406209926358, 91.19699162172292],
 [879.1656602459713, -581.9032177447318, 91.21245004565435],
 [879.196849813, -581.265461848, 91.2209560929]]
curve.knotvector = [0.0, 0.0, 0.0, 0.5, 0.5, 1.0, 1.0, 1.0]
curve.weights = [1.0, 0.7071067811865476, 1.0, 0.9999796117372515, 1.0]

curve.vis = VisMPL.VisCurve3D()
curve.render()

# Elevate curve degree
degree_elevated = operations.degree_operations(curve, [1])
degree_elevated.vis = VisMPL.VisCurve3D()
degree_elevated.render()

Expected Behavior Degree elevation operation should not modifz the shape of the input curve.

Configuration:

• OS: Windows 10
• Python distribution: Anaconda
• Python version: Python 3.6.10
• geomdl install source: Anaconda
• geomdl version/branch: 5.3.1, py_0

Screenshots (Optional)