SampledDensityMap should natively handle core::Gaussians

[cgreenberg]

Currently if you create a SampledDensityMap it treats all Gaussians as point particles and just uses whatever kernel was requested. I will add a CoreGaussianKernel which directly uses the covariance matrix while sampling the density.

[cgreenberg]

An alternative is to just adapt the existing GaussianKernel so that if the particle is a core::Gaussian, use its covariance directly. I like that better.

[benmwebb]

I also like that better. ;)

[cgreenberg]

Hmm. I don't want to reproduce functionality from algebra::get_rasterized() which makes a density map out of a bunch of Gaussian3Ds. It works slightly differently because you only want to calculate the Gaussian3D's inverse covariance once. Maybe we can change the loop in SampledDensityMap or cache the inverses...