Test accuracy of metric finite differencing formulas
We can probably make this a unit test...
- Vary the metric.DELTA variable to check e.g., the volume element of the computed metric. There should be a region where the result seems stable. Bonus points if you can figure it out from first principles
- make sure that it works for any input and any coordinate system. NOTE - the correct answer is almost certainly a function of the coordinates itself (I think)