Add gdal vector collect, gdal vector dissolve

[dbaston]

Adds gdal vector collect, an opposite operation to explode-collections. The --group-by option is used to control which features get combined.

Also adds gdal vector dissolve, which performs UnaryUnion on individual features. I think the name "Union" can be confusing (easily mistaken for https://pro.arcgis.com/en/pro-app/latest/tool-reference/analysis/union.htm)

On the other hand, while "dissolve" captures the behavior for polygon and maybe point inputs, I don't think it's a good description of how lines are handled. (They're noded, but not combined.) I don't know if "dissolve" should apply GEOSLineMerge (https://postgis.net/docs/ST_LineMerge.html) to the outputs of UnaryUnion (thus "dissolving" the linestrings), or if I should just rename the operation to gdal vector unary-union or gdal vector union.

[coveralls]

coverage: 71.359% (-0.001%) from 71.36% when pulling 2b5fe2e0d6e2a57b9dafdd55d3f36f08ccc6ec85 on dbaston:collect-dissolve into ea75dfb98cb6a6fe27d826f840178e8e6b6fe626 on OSGeo:master.

[rouault]

I don't know if "dissolve" should apply GEOSLineMerge (https://postgis.net/docs/ST_LineMerge.html) to the outputs of UnaryUnion (thus "dissolving" the linestrings)

That would probably be reasonable. Looking at QGIS dissolve algorithms in https://github.com/qgis/QGIS/blob/0ceeea4a5de28b3644392b7f0b10be220419ee5f/src/analysis/processing/qgsalgorithmdissolve.cpp#L281 , it calls QgsGeometry::mergeLines(), which I assume must do something similar to GEOSLineMerge, if the output of the unary union is linear

[dbaston]

I don't especially like GEOS' approach of silently dropping non-linear geometries, so either GDAL will need to do some careful filtering of what it provides to GEOSLineMerge, or I'll need to expose a new LineMerge in GEOS that doesn't have that behavior.

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