Add Exponential of Semicircle Window

[tknopp]

We want to integrate the "exponential of semicircle" window proposed in this paper: https://arxiv.org/abs/1808.06736
I am not completely sure but my FINUFFT wrapper indicates that it is slightly more accurate than Kaiser-Bessel.

Remarks:

• The window is completely simple
• Its Fourier transform seem to have no analytical form and therefore it is approximated using Gauss-Legendre-Quadrature (equation 3.10 from the paper)
• The implementation in FINUFFT is here: https://github.com/flatironinstitute/finufft/blob/master/src/finufft.cpp#L227. But if I get it right they applied some caching tricks that result in this https://github.com/flatironinstitute/finufft/blob/master/src/finufft.cpp#L167 But these implementations should only be used for debugging purpose, the mathematics is straight forward and permits a cleanroom implementation.
• Both functions require the Gauss-Legendre nodes, which are calculated here: https://github.com/flatironinstitute/finufft/tree/master/contrib Of course we would not use that but instead go for a Julia package. A quick search pointed me at https://github.com/JuliaApproximation/FastGaussQuadrature.jl

This would be a nice "good first time contributor" project if somebody want to have look at it. Otherwise I will five it a go when time permits.

[tknopp]

Apparently, I was wrong, which is actually a good thing. Kaiser bessel is still more accurate than the semicircle window and it has an analytical Fourier transform. So the motivation to implement this is now more of an academic nature to make accuracy comparisons. The evaluation time for the window does not really matter since the polynomial approximation of the window that we use in the new precompute option is accurate and very fast.