Use Eigen3 in preference to Eigen2 if found, but in support mode.

Eigen3 allows a staged migration to the new API. This patch enables
building with either Eigen2 or Eigen3 with complete compatibility.
Future versions of Avogadro can use Eigen3 directly. In the meantime,
both version 2.x and 3.x are now supported.

Change-Id: Ibae3a7ca719124af73f52a19244beb9a2cbf180e
Merge topic 'ENH_eigen3-support' into master

3cc14da0 Use Eigen3 in preference to Eigen2 if found, but in support mode.