Make ASTRAL submodule

I removed ASTER from /addons and added it as an actual git submodule. The implication of this is that users need to compile astral-pro themselves. I updated the consensus_tree function to give compilation instructions. Also, users need to clone this git with git clone --recursive now. I added this to the README.md. NB: Currently, we do not use the most recent version of ASTRAL-Pro. I did try the most recent version, but it cannot handle polytomies anymore. I am still looking into this to make sure we will use the most recent and functional version.

Finally, I also removed all (non-functional) unit tests from the repo and adjusted the Maven compilation instructions accordingly.