Add Eigen 3.4.0

Eigen (at 3.4.0/3147391d) has been added to support Vapour Transport functionality within the SNOWPACK project.

See discussion at https://code.wsl.ch/snow-models/snowpack/-/merge_requests/8

requested review from @bavay

assigned to @evan.thomas

approved this merge request

mentioned in commit 4eb673be

merged