Spherical isometries, triangular block-banded eigensolvers, generalized Zernike and Dunkl-Xu OPs on the disk

This release:

• adds support for spherical isometries (rotations and reflections and general orthogonal coordinate transformations of spherical harmonic expansions), thanks in part to @ikaroruan.
• includes an implementation of the factored alternating direction implicit method for diagonal matrix equations with low-rank right-hand sides, thanks in part to @BrockKlippenstein.
• adds methods for the complete elliptic integrals K(k) and E(k) and Jacobian elliptic functions sn(x,k), cn(x,k), and dn(x,k).
• converts associated classical orthogonal polynomial connection problems from two parameter triangular banded eigenvalue problems to quadratic triangular banded generalized eigenvalue problems.
• adds fast divide and conquer methods for block 2x2 triangular-banded generalized eigenvalue problems (for linearizations of the QEPs above).
• generalizes Zernike polynomials to a real orthonormal basis for L²(D², r^2α+1(1-r^2)^β dr dθ), a breaking change to the API.
• adds Dunkl-Xu polynomials on the disk, a real orthonormal basis for L²(D², (1-x^2-y^2)^β dx dy), updating the holomorphic.c example to show the duality of the approaches.