Multitaper method

[pwstegman]

Multitaper method for power spectral density estimation.

Implemented in ~v1.8~. Need to add unit tests.

Edit: Moved to v1.9

For computation of discrete prolate spheroidal (Slepian) sequences, a length up to 128 is calculated. Lengths past 128 are linearly interpolated up.

For tapers with near-unity eigenvalues for common time half bandwidths (2.5, 4, etc.) the curves are smooth enough that linear interpolation works well. A more efficient eigenvector method will help remove the need to interpolate.

[pwstegman]

Moved to v1.9 to allow time for adding support of cubic spline interpolation of discrete prolate spheroidal (Slepian) sequences