bettermoments: A robust method to measure line centroids

Initial release of `bettermoments`, a robust way to measure the centroid of a line profile for sources believed to be well described by a single kinematical component.

As an alternative to traditional 'first moment maps' which take the intensity weighted averaged velocity (or frequency), `bettermoments` finds the pixel of maximum intensity and fits a quadratic to this pixel and the two neighboring pixels. It has been shown in the context of PSF fitting this that approach yields near-optimal precision.