Add eigen3 support

See nicadist#6 (closed) for new installation method (coming up!)

and for current (legacy) method:

Fedora:
https://src.fedoraproject.org/rpms/eigen3

Ubuntu:
https://packages.ubuntu.com/bionic/libeigen3-dev

Debian:
https://packages.debian.org/stable/source/eigen3

CentOS7:
https://centos.pkgs.org/7/epel-aarch64/eigen3-devel-3.3.4-6.el7.noarch.rpm.html

Mint:
https://community.linuxmint.com/software/view/libeigen3-dev