Usage:
    USES=eigen:<version>[,<type>]

    version: 2 or 3 (required)
    type:    build (default), run

  For example:
    USES=eigen:2,build,run
  will add a BUILD- and RUN_DEPENDS on math/eigen2, and
    USES=eigen:3
  will add a BUILD_DEPENDS on math/eigen3.

* Convert the existing ports to use it
  - biology/iqtree: remove run time dependency (seemed not to be needed)
  - graphics/movit: remove run time dependency (seemed not to be needed)
  - science/avogadro: add run time dependeny (installed cmake file requires it to be present)

Reviewed by:	rakuco, mat
Differential Revision:	https://reviews.freebsd.org/D13702