Add 'gdal vector concat' (port of Python ogrmerge script)

[rouault]

• Can be used standalone or as the first step of "gdal vector pipeline"
• Strongly inspired from Python ogrmerge, adding a 3rd mode (which is the default) to fully control how source layers are grouped together.

[jratike80]

That "cat" is short for "concatenate" is probably obvious for Linux users, In SQL it is "concat" and for me it feels somehow more friendly.

[nyalldawson]

That "cat" is short for "concatenate" is probably obvious for Linux users

To me "cat" would be a dump of the contents, like how the Linux cat command is most often used to view file contents. I agree that "concat" would be less ambiguous.

[rouault]

Renamed to concat