Provide CLI script

[clausnagel]

The recommended way to use the Importer/Exporter in CLI mode is to invoke the impexp-client.jar using the java tool. From the doc:

$ java -jar lib/impexp-client-{version}.jar [-options]

This is neither intuitive nor handy and it also faces disadvantages. For example, you cannot easily put this command on your PATH and the {version} part of the command must be adapted with every new release. In theory, you could use the provided start script 3DCityDB-Importer-Exporter(.bat) to run the Importer/Exporter in CLI mode. But at least for Windows, the start script uses "start /min cmd /c" to execute the tool. This is OK and makes sense for launching the graphical user interface but suppresses all output when invoking the script from a shell.

This PR proposes to have an explicit CLI script impexp(.bat) in addition to the start script. The CLI script is located in the bin subfolder. This makes using the Importer/Exporter in CLI mode really simple. For example, the following command shows the help message and all available options:

$ impexp -help

This way, the CLI script can be put in the PATH and is independent of the release version. And the start script can be re-implemented such that it just invokes the CLI script with the option to launch the GUI.

[clausnagel]

Discussed offline with @yaozhihang. Merging.