New USES=eigen

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