[Test] Fix octree construction

  Position of octree is defined by resolution only. cell center coordinates are
  of the form:
    ((kx+.5)*resolution, (ky+.5)*resolution, (kz+.5)*resolution), where
    kx, ky, kz are integers.
  This commit fixes the construction of the environment octree in order to
  respect this constraint.
  In test_fcl_octree, envOctree contains envMesh.
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passed robotpkg-hpp-fcl-16.04-debug #12422

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passed robotpkg-hpp-fcl-16.04-release #12421

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passed robotpkg-hpp-fcl-18.04-debug #12424

00:42:12

passed robotpkg-hpp-fcl-18.04-release #12423

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