Added documentation to the GeoTiff* files - #1560

@lossyrob These are the methods which I'm not really getting right now: 1, 2, 3, and 4.

I know they have something to do with macros, but I don't understand it well enough to explain it in documentation.

@MyNameIsBouf

These functions implement the interface methods of IterableTile and MappableTile[Tile], which are extended by the Tile trait (https://github.com/geotrellis/geotrellis/blob/master/raster/src/main/scala/geotrellis/raster/Tile.scala#L34)

MappleTile extends MacroMappableTile (https://github.com/geotrellis/geotrellis/blob/master/raster/src/main/scala/geotrellis/raster/MappableTile.scala#L8), and the implementation of function parameters that take more than 2 args is called via a macro (https://github.com/geotrellis/geotrellis/blob/master/raster/src/main/scala/geotrellis/raster/MappableTile.scala#L15). The IterableTile does a similar thing.

The macro methods create implementations of map and foreach that are called with "Visitor" or "Mapper" traits. These traits are new-ed up at compile time by macros like this one (https://github.com/geotrellis/geotrellis/blob/master/macros/src/main/scala/geotrellis/macros/TileMacros.scala#L34).

We don't need this crazy Macro business for function parameters with 2 or less parameters because of specialization. See how Function2 has specialized type params (https://github.com/scala/scala/blob/2.12.x/src/library/scala/Function2.scala#L29), but Function3 does not (https://github.com/scala/scala/blob/2.12.x/src/library/scala/Function3.scala#L16). Specialization is a weird and complex topic; here's an article dealing with specialization: http://axel22.github.io/2013/11/03/specialization-quirks.html

Macros are another heavy topic. Here's some docs on them: http://docs.scala-lang.org/overviews/macros/overview.html

Let me know if you have followup questions after trying to digest some of that stuff.

@lossyrob Travis failed, but after looking at what caused the problem and double checking my code, I think there's actually nothing wrong with it. The failed test had something to do with PostgreSQL, which doesn't have anything to do with what I've done.

@jbouffard make sure to mark PR's that are ready for review (and merging if it gets a +1) as not [WIP]

@lossyrob Okay. I made the changes to both the documentation and to the title.

👍