tetgen as a submodule.

Tetgen 1.5.0 is added as a git submodule. This module is automatically checked out when `OGS_BUILD_GUI=ON` and the executable also gets packaged with the Data Explorer.

This addition allows us to distribute tetgen along with the Data Explorer easily. Shipping the Data Explorer with tetgen included as a binary should be a problem as I understand the license but we should add some acknowledgments (e.g. in the About-dialog) about it.

The tetgen source is hosted at https://github.com/ufz/tetgen.git

This is also related to #399.