With MR !22073 we dropped the support to Eigen <= 3.3.0 but, apparently, we are still building 3.2.9 as part of Externals.
This MR changes the version to package 3.3.7 from LCG.
Also, this MR updates the README file of the VP1LightExternals project.
pinging @tsulaia , @fwinkl , @akraszna