feat: add custom result objects to integrate_strain_profile

[talledodiego]

This is a proposal for adding a custom result object also for method integrate_strain_profile. In this case two different results are available: IntegrateStrainStiffnessResult when integrating modulus, and IntegrateStrainForceResult when integrating stresses.
They both have methods like asarray for having the array representation.

Optionally we could evaluate if needed to implement

def __iter__(self):
    yield self.n
    yield self.m_y
    yield self.m_z

def __len__(self):
    return 3

def __getitem__(self, i):
    return (self.n, self.m_y, self.m_z)[i]

But I have the feeling it might be less transparent.

[mortenengen]

Thanks for this useful contribution!