Add convenience functions for symmetric eigen BandedMatrices

[jishnub]

This provides a convenient function for the Aurentz-Slevinsky eigenvalue problem symmetrization.

After this

L = -Derivative(Legendre(0..1))^2
B = Dirichlet()
S = ApproxFun.SymmetricEigensystem(L, B);
λ = eigvals(S,20); # return 20 eigenvalues

[codecov]

Codecov Report

Patch coverage: 97.61% and project coverage change: +5.72 🎉

Comparison is base (803d0fc) 64.98% compared to head (a3ca431) 70.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
+ Coverage   64.98%   70.71%   +5.72%     
==========================================
  Files          28       29       +1     
  Lines        2622     2667      +45     
==========================================
+ Hits         1704     1886     +182     
+ Misses        918      781     -137     

Impacted Files	Coverage Δ
src/ApproxFunOrthogonalPolynomials.jl	81.48% <ø> (ø)
src/symeigen.jl	97.29% <97.29%> (ø)
src/Spaces/Jacobi/Jacobi.jl	68.18% <100.00%> (+2.46%)	⬆️

... and 15 files with indirect coverage changes

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