Add more density compensation schemes

[paquiteau]

Density compensation weights act as preconditionner on the adjoint operator, and leads to better image quality (and thus faster convergence in iterative schemes)

Currently we support Voronoi estimation and Pipe's iterative scheme. Yet those estimation are simple heuristic, and does optimize a criteria, thus there is no way of knowing if its "best" in one form or the other. Also the current Voronoi method implementation is not applicable in 3D (too much memory and computation requirement).
Jeff Fessler's work: https://web.eecs.umich.edu/~fessler/book/c-four.pdf provides a good overview of the available methods (see notably section 6.4.2)

Implementation of these methods would be a great asset to mri-nufft, as they can be made agnostic to the NUFFT backend. Most of them are iterative, so a benchmark using benchopt could also be done.

[paquiteau]

Cell-Couting (#66, #68) has been added, as well as a fix for Outlier in Voronoi(#78).