TileDB: add support for building overviews on rasters stored as TileDB groups

[rouault]

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Supports the CREATE_GROUP=YES creation option to create TileDB raster datasets as a TileDB group, to make it cleaner to store overview within it.

See https://github.com/rouault/gdal/blob/tiledb_overview/frmts/tiledb/overview_model.rst for a description on how TileDB overviews are stored.

@normanb / @sgillies : this is mostly based on https://docs.google.com/document/d/1B-NizsoJn79W_RV3ujg6K1D5xJmWEja29Bv3323uYCU/edit#heading=h.p8z7kqfrb9st (with the removal of creating overviews on non-group dataset)

[rouault]

@sgillies @normanb Did you sync together regarding Sean's above suggestions?

[sgillies]

@rouault sorry for the delay! We did discuss and here are the conclusions:

1. Overview arrays should be named l_0, l_1, etc.
2. Dataset_type metadata should remain raster.
3. We do not desire a sidecar .ovr group and the TileDB platform will not support it, so it would be best to delete that format variant.
4. We'd like to see CREATE_GROUP=YES be the default. Groups will be favored by the TileDB platform. We understand that this means a group with a single array for users who don't request overviews.

[rouault]

We did discuss and here are the conclusions:

I've updated the pull request with those changes, starting at commit "Revert "TileDB raster: implement BuildOverviews() for base dataset not in group mode"). As the CREATE_GROUP creation option is common to the raster and vector sides of the driver, I found it to be perhaps simpler to change its default to YES for both (at least to be able to present a default value in the XML metadata of GDAL_DMD_CREATIONOPTIONLIST), so I also changed it for vectors. I could revert that change if needed.

[coveralls]

coverage: 69.137% (+0.001%) from 69.136% when pulling 2159d27f3f1ee084f612b95d31c61a4bd651859f on rouault:tiledb_overview into 983c3bd3b32b33ad11a9e2e0f4402a1f6965cd42 on OSGeo:master.

[rouault]

As the CREATE_GROUP creation option is common to the raster and vector sides of the driver, I found it to be perhaps simpler to change its default to YES for both (at least to be able to present a default value in the XML metadata of GDAL_DMD_CREATIONOPTIONLIST), so I also changed it for vectors. I could revert that change if needed.

@sgillies Any opinion regarding that?

[sgillies]

@rouault we would prefer to not change the default for vectors now.

[rouault]

@rouault we would prefer to not change the default for vectors now.

ok, change for vectors reverted

[sgillies]

Thank you @rouault ! I think this looks great.