Refactor

[haakoek]

• Fix sign for boundary elements of D1 in Legendre-Lobatto basis. The expression for D1[0,0] and D1[N,N] given by Eq.(F.55) in Ref.[1] has the wrong sign as opposed to the expression (3) for d_ij in Ref.[2].
• Add tests for differentiation matrices, numerical integration of polynomials and $f(x) = x^k exp(-ax^2)$, and eigenvalues and eigenfunctions for the 1D particle in box and harmonics oscillator using the Gauss-Legendre-Lobatto grid.
• Remove unused scripts and modules from the spherical_coordinates directory.
• Remove unused functions in spherical_coordintates/utils.py. In particular, remove functions that used numba. Thus, there should no longer be necessary to have numba installed for the package to work.
• Import gauss_legendre_lobatto from pseudospectral_grids in all examples.

[1]: Boyd, John P. Chebyshev and Fourier spectral methods. Courier Corporation, 2001.
[2]: https://doc.nektar.info/tutorials/latest/fundamentals/differentiation/fundamentals-differentiationch2.html