Rework CLI options for KML/COLLADA/glTF export

The following points have been raised in the discussion of #147:

• merge --display-mode and --visible-from into one option
• add options for the elevation settings for KML/COLLADA/glTF exports
• add explicit CLI options for the collada2gltf converter and skip -O was: clarify how to pass parameters to the collada2gltf converter

Check out the export-vis-cli branch to follow and test the changes.

[clausnagel]

I merged the --visible-from option into the --display-mode option.

I removed the -O option and added --gltf-version, --gltf-embed-textures, --gltf-binary and --gltf-draco-compression as explicit options to control the behaviour of the collada2gltf converter. Note that the CLI supports abbreviated option names as explained in the picocli online documentation. Thus, for example, you don’t have to type the full option name --gltf-draco-compression but can also use --gDC. Much simpler 😃 This works for all options of all commands.

I also added options to control the elevation and mainly followed the proposal of @Son-HNguyen in #139 for this.

The export-vis is now a monster command (just print out the help). But, well, the feedback shows we obviously need all those options. So, I am thinking to also add the COLLADA rendering options as proposed by @Son-HNguyen in #139 (see [rendering_options]).

The changes are availble in the export-vis-cli branch. @yaozhihang, @BWibo, @Son-HNguyen: would be great if you could do some testing.

[yaozhihang]

I like your proposal of dropping the -O option and introducing three options --gltf-version, --gltf-binary, and --gltf-draco-compression instead. This makes the CLI interface clearer and easier to understand.