Topography data

This is a placeholder for the topography data used by the test suite. Note that those should be automatically downloaded and installed by CMake. Alternatively, you can do this manually as:

cd /path/to/install/share
wget https://github.com/niess/turtle-perfs/releases/download/topography/topography.tgz
tar -xvzf topography.tgz && rm -f topography.tgz