Add bending moment and shear force calculation to py_analysis functions

[edoglione]

Hi, thank you for this library, I find it very useful for lateral pile analysis.
I noticed that py_analysis_1_SI and py_analysis_2_SI currently return only lateral displacements y and node depths z. For practical engineering use, bending moment M(z) and shear force V(z) are equally important outputs.
I have implemented these by numerical differentiation of the displacement profile:

d2y = np.gradient(np.gradient(y, z), z)
M = EI * d2y   # bending moment (N·m)
V = EI * np.gradient(d2y, z)  # shear force (N)

Would you be open to including this in the main functions?

I would be happy to submit a Pull Request if you are interested.

[asitha-sena]

Thanks for pointing this out, @edoglione. I agree that BM and SF are important so I'm happy to accept a pull request.