Add ability to label individual positions along an axis

It would be nice if rather than only an index, each position along each dimensional axis could be labeled with a string as well. A very common use case is to have multiple wavelengths labeled with the stain used; e.g., labeling a GFP fluorescence channel as "GFP" or "509nm".

In ImageJ 1.x these are called "slice labels" and you can have one per image plane.

Right now, SCIFIO has some [hacks to read and propagate them](https://github.com/scifio/scifio/blob/scifio-0.17.1/src/main/java/io/scif/formats/TIFFFormat.java#L690-L696). But `ImgPlus` et. al needs to properly support it.

Migrated-From: http://trac.imagej.net/ticket/1567

Start by reading the linked SCIFIO TIFFFormat slice-label handling and the ImgPlus references in the issue. Determine how labels should be represented for each dimensional axis, then verify that individual positions such as GFP or 509nm can be stored and propagated instead of only using indices.

Written by the indexing model from the issue text.