Gaussian filtering when using `pole_density_function`

[harripj]

When providing the argument sigma > 0 to pole_density_function we currently convolve a gaussian to the distribution histogram. Internally pole_density_function uses an equal area sampling grid on S2 to ensure constant bin areas, however this means that the bin spacing along the latitudinal direction is not consistent and a fixed size gaussian kernel is not 100% appropriate.

An alternative would be to sample the histogram initially on an equal angle (uv) grid, apply the gaussian filter on the equal angle grid, and resample this grid to the equal area grid for quantitative analysis. This is possible but leads to aliasing in the final distribution.

Another alternative would be compute a functional representation of the distribution and apply the blurring in the frequency domain before resampling, this may also help with the pole distortion raised in #441.