Add Smoothing Methods for pyrolite.plot.density

[morganjwilliams]

Currently, pyrolite.plot.density provides either histogram methods (which are occasionally noisy) or evaluation of a kernel density estimate over a grid (which are typically over-smoothed, especially for multimodal data). The histogram methods could be improved by providing options for smoothing methods - either Gaussian or k-nearest neighbors (e.g. see https://stackoverflow.com/a/47350350; https://stackoverflow.com/a/59920744/12198281).