Better handling of samples

[aimalz]

Currently, qp treats all parametrizations the same in calculating metrics (and stacking) by forcing evaluations on a grid, which doesn't always make sense, particularly for samples. At the very least, there should be more options for user control of that process, and ideally, there should be a way around it by calculating metrics from the native format without an extra approximation. (This would also speed up calculating metrics for the histogram representation, for example.)