fix: Update GenericSection to accept SurfaceGeometry

[talledodiego]

Update GenericSection to work also with SurfaceGeometry as input.

This fix an issue mentioned in #113

# Materials
concrete = ConcreteEC2_2004(25)
# Create section
poly = Polygon(((0, 0), (350, 0), (350, 500), (0, 500)))
geo = SurfaceGeometry(poly, concrete)
# Section from SurfaceGeometry
sec = GenericSection(geo)
print(f"Area = {sec.gross_properties.area*1e-2} cm2")

# Section from CompoundGeometry
sec = GenericSection(CompoundGeometry([geo]))
print(f"Area = {sec.gross_properties.area*1e-2} cm2") 

now both definition works and the output is:

Area = 1750.0 cm2
Area = 1750.0 cm2

[mortenengen]

Nice fix 👍