Commonize special polynomials

[ohno]

We currently prioritize each model being independent and self-contained. Some orthogonal polynomials can be made common. Reconsider whether the code should be common before updating to v1.0.0. This issue is related to #49.

[lhapp27]

I think the same holds true also for the spherical harmonics. All (spherical symmetric) 3D problems they can use the same common function.

[ohno]

I gave priority to that all member functions are self-contained in each file.

Also, Commonization of spherical harmonic functions, etc. does not change　the Public API. Therefore, commonization can be done after the release of a major version 1.0.0 if we need.