Taper Function, Problems when mesh has offset

Description

In the openaerostruct/geometry/utils.py module the Taper Function, taper, that is implemented produces wrong results when the mesh has an offset in the y (spanwise) direction.

Steps to reproduce issue

1. Use the generate_mesh function available in the same module to produce a rectangular wing with an offset in the y direction:

from openaerostruct.geometry.utils import (
generate_mesh,
taper
)

mesh_dict = {'num_y' : num_y_outboard,
'num_x' : num_x,
'wing_type' : 'rect',
'symmetry' : True,
'span_cos_spacing' : 0.5,
'span' : 2.5,
'root_chord' : 0.399,
'offset' : np.array([0, -0.75, 0])
}

mesh = generate_mesh(mesh_dict)

3. Use the taper function to change the geometry of the mesh:

mesh = taper(
mesh=mesh,
taper_ratio=0.55,
symmetry=True
)

Current behavior

mesh[0, -1, 0] returns a value for the x-coordinate of the Leading Edge root different than the original value defined when calling the generate_mesh helper function.

mesh[-1, -1, 0] also returns a different value than previously defined by the generate_function helper function.

Expected behavior

mesh[0, -1, 0] and mesh[0, -1, 0] to return the same value as given by the generate_function helper function.
The x-coordinates of the wing root should be the same after calling the taper function.

Code versions

• Python 3.9.5
• OpenMDAO 3.6.0
• OpenAeroStruct 2.3.0

Start in openaerostruct/geometry/utils.py by reproducing the issue with generate_mesh using the y offset, then applying taper with symmetry enabled. Compare the root leading-edge and trailing-edge x-coordinates before and after taper; the work is done when the wing-root x-coordinates remain unchanged.