load urls from existing urls directory by default

[nhemsley]

I had some issues with copying the source urls, it wasn't obvious where to copy the urls to.

This is somewhat opinionated, but just defaults to loading the urls from the PROJECT_ROOT/urls directory

[kurtkuehnert]

Thanks for the PR!
Currently, the urls file should be copied into the directory of the terrain you want to download.
Something like this: terrain_dir/MyTerrain/urls.txt.

I have considered using the working directory, but this causes some inconsistencies when the program is not run from the command line, but executed from the file explorer.

I will clarify the usage. 👍