Nonequidistant coordinates

[thchr]

At present, isosurface implicitly assumes that inputs X, Y, and Z are equidistantly spaced. There's no real reason to assume this afaik however. At minimum, it's not great that the assumption is implicit and conversion occurs without warning/error.

Non-equidistant coordinates can e.g. be helpful to simulate isocaps in combination with a jump-function at boundaries.

[thchr]

I'm happy to make a PR if @sjkelly are open to merging a change like this?

It would just be adding an if case to use X, Y, and Z as-is in the case where X, Y, and Z inputs to isosurface are AbstractVectors & adding a docstring comment that if they are not, that they will be assumed equidistant.