Merge georust#447

447: Maintenance cleanup on `dataset.rs` r=lnicola a=metasim

- [X] I agree to follow the project's [code of conduct](https://github.com/georust/gdal/blob/master/CODE_OF_CONDUCT.md).
- [X] I added an entry to `CHANGES.md` if knowledge of this change could be valuable to users.
---

Changes: 
* Moved ancillary types from `dataset.rs` into their own or related files.
* Moved non-core Dataset methods into respective `raster` or `vector` modules. Aims to simplify maintainability via "separation of concerns".
* Removed `unsafe` from pointer accessor methods.


_Possibly_ breaking changes:

* Moved `LayerIterator`, `LayerOptions` and `Transaction` to `crate::vector`.

---

Aside: The impetus for this refactor is to focus the changes necessary for adding `Dataset` to georust#445. But even if that PR takes a different route, this change will benefit long-term maintenance.

Co-authored-by: Simeon H.K. Fitch <fitch@astraea.io>

.gitignore

@@ -4,6 +4,7 @@
 /gdal-sys/target
 /.idea
 /3rd
+/fixtures/tinymarble.tif.aux.xml
 
 # gtags
 GPATH

CHANGES.md

@@ -2,6 +2,12 @@
 
 ## Unreleased
 
+ - Refactored `dataset.rs` to focus on core `Dataset` operations, moving ancillary types and operations to other files.
+ - **Breaking**: Moved `LayerIterator`, `LayerOptions` and `Transaction` to `crate::vector`.
+ - Accessors `MajorObject::gdal_object_ptr` and `Dataset::c_dataset()` are no longer marked as `unsafe` (it's not idiomatic Rust to do so).
+
+   - <https://github.com/georust/gdal/pull/447> 
+
  - Fixed build script error with development GDAL versions