FMM Fast Chebyshev--Legendre transform

Thanks to @mikaem for #83!

Store rho in annulus synthesis and analysis

This allows construction of adjoint plans to get the value of rho.

Annulus transforms

• Change the hard exits in the modified plans to warnings.
• Add single-precision modified plan methods.
• Add annulus transforms and documentation and an example.

Modified classical orthogonal polynomial transforms

This computes the connection coefficients between polynomially- or rationally-modified orthogonality weights of the classical orthonormal polynomials and the unmodified ones using banded Cholesky and QL factorizations.

Allow FFTW planner flags into fftw.c API

JuliaApproximation/FastTransforms.jl#163

Fix FFTW usage

Solves JuliaApproximation/FastTransforms.jl#162

Improve spherical harmonic rotations

Thanks to @mreineck for #69 and #71.

Fix spin-weighted spherical harmonics with |m| = 1

This fixes JuliaApproximation/FastTransforms.jl#139

Support adjoint and transposes for reverse-mode automatic differentiation

v0.5.0

add const char TRANS to remaining fftw transforms

Fix associated transforms with eigenvalues with algebraic and geometric multiplicity greater than 1

Use the true eigenvectors to correct the singularities in the low-rank block V₁₂ = V̂₁₂ + S₁₂, where S₁₂ is the sparse correction.