PR 2301 and follow up (rasterio#2318)

* Add in memory raster that subclasses DatasetBase.

* Remove unused variables.

* Add r+ to modes setting georeferencing.

* Fix dtype argument.

* Use InMemoryRasterArray

* Use InMemoryRasterArray in warp.

* Eliminate unnecessary copy.

* Add missing word.

* Use InMemoryRasterArray in fillnodata

* Add array interface method.

* Resolve fillnodata test failure.

* Remove unnecessary copy.

* Cleaner array handling.

Make sure _array is always an array, but only copy when needed/wanted.

* Rename InMemoryRaster to MemoryDataset.

* Add internal use only comment to MemoryDataset.

* Follow ups on rasterio#2301

* Fix parameter type in docstring and whitespace

Co-authored-by: Ryan Grout <ryan@ryangrout.org>

Rounding like gdal-merge as suggested by @groutr (rasterio#2311)

* Rounding like gdal-merge as suggested by @groutr

* Use only GDAL's rounding convention everywhere in rasterio

* Eliminate old window rounding implementations

This is 1.2.10

This is 1.2.9

This is 1.2.8

This is 1.2.7

This is 1.2.6

Resolves rasterio#2218

Remove test accidentally committed

Update mock_debian fixture