Fixed broken eigen download link in the install script

[MatthiasKi]

The provided link to the eigen library (http://bitbucket.org/eigen/eigen/get/3.3.7.tar.bz2) in the install script is a dead-end. I replaced it (https://gitlab.com/libeigen/eigen/-/archive/3.3.7/eigen-3.3.7.tar.bz2), using the same eigen version as originally and adjusted the install script accordingly.