Propose moving color-related types from rasterband.rs into new color.rs file.

[metasim]

The rasterband.rs file covers an arguably broad set of concerns. I want to propose moving the types selected below into a new module.

In addition to this, two other questions might be worth pondering:

1. Should the refactored types be exposed as:
   a. pub use color::* in raster (to preserve binary compatibility, e.g. gdal::raster::ColorTable)?
   b. new paths (e.g. gdal::raster::color::ColorTable?
2. Should statistics-related items also be moved to a separate module?

[michaelkirk]

a. pub use color::* in raster (to preserve binary compatibility, e.g. gdal::raster::ColorTable)?

If I'm understanding, the way I'd probably do this is:

1. move things to their new home.
2. re-export them from the legacy location so nothing breaks for people.
3. mark the legacy re-exports as deprecated so people can update their code when it's convenient for them.

Steps 1-3 can be done in a minor (non-breaking) release.

4. eventually (e.g. 6 months after release) delete the deprecated re-exports in a breaking release.

[metasim]

Great idea on deprecating the re-exports.