Create position-sensitive map operation

Nobody has claimed this yet.

Would it make sense to create a map operation where the position within the `Interval` (or `RealInterval`) is known to the nested function? We would probably need a new subtype of `Function` that has the position (maybe as a `double[]`) as an additional argument to the `compute` method.

I recently created an [equation op](https://github.com/imagej/imagej-ops/blob/faec4dcda7172c804eb1ec816865654594b77759/src/main/java/imagej/ops/equation/DefaultEquation.java#L78) that illustrates why such a map would be nice. The implementation currently uses an `IterableInterval` as its output, but the actual operation is completely independent pixel-wise. However, the value of each pixel is defined by an equation which operates on the `Interval` position, so that you can generate stuff like gradients, sinusoidal shapes, or whatever your mathy brain can dream up. So at the moment, it is not possible to use the current mappers to create an equation op that operates on a single pixel only.

I think this new mapper would end up being quite useful; it is sort of analogous to ImgLib2's distinction between "cursor" and "localizing cursor". Some operations need to localize at every pixel, some occasionally, and some never. Shall OPS support all of these cases, too?

/cc @dietzc

Start with the linked equation implementation in src/main/java/imagej/ops/equation/DefaultEquation.java and compare it with the current mapper and Function APIs. Determine how a position argument should be represented and which cursor-localization cases the operation must support. Done means a decided API and an implementation path for position-aware mapping, with coverage for equation-style pixel-wise operations.

Written by the indexing model from the issue text.