fix: rotate strain in result from bending strength

[talledodiego]

This PR solves the bug spot by @MestreCarlos in #163.

Now the following code snippet provided by @MestreCarlos:

import math
from pprint import pprint

from shapely import Polygon

from structuralcodes.geometry import SurfaceGeometry
from structuralcodes.materials.concrete import ConcreteEC2_2004
from structuralcodes.materials.reinforcement import ReinforcementEC2_2023
from structuralcodes.sections._generic import GenericSection
from structuralcodes.geometry import add_reinforcement_line

# Materials
concrete = ConcreteEC2_2004(25)
reinforcement = ReinforcementEC2_2023(
    fyk=500, Es=200000, density=7850, ftk=550, epsuk=0.07
)

# Create section
poly = Polygon(((0, 0), (350, 0), (350, 500), (0, 500)))
geo = SurfaceGeometry(poly, concrete)
geo = add_reinforcement_line(
    geo, (50, 450), (300, 450), 20, reinforcement, n=10
)
geo = add_reinforcement_line(geo, (50, 50), (300, 50), 20, reinforcement, n=10)
geo = geo.translate(-175, -250)
sec = GenericSection(geo)

# Draw & print
print('====== SECTION ======')
print('====== Theta = pi/6 ======')
pprint(sec.section_calculator.calculate_bending_strength(theta=math.pi / 6))

produces the following expected result:

====== SECTION ======
====== Theta = pi/6 ======
UltimateBendingMomentResults(theta=0.5235987755982988,
n=-0.003881820593960583,
m_y=-537978185.0265349,
m_z=-64913391.04673183,
chi_y=-1.4516931240180188e-05,
chi_z=-8.381354159325317e-06,
eps_a=0.0015959697879269737)

[mortenengen]

Thanks for providing this fix!