[PMP] Ceres requires exact matching Eigen version

[soesau]

Ceres requires the exact Eigen version that has been used for compiling the Ceres library.

Thus requiring recompilation of Ceres after every update of Eigen.

Concerns PMP::angle_and_area_smoothing().
See TestReport_lrineau_Ubuntu-latest-GCC6-Release.gz.

[afabri]

So we have to compile Ceres with the Eigen we use in the testsuite?

[soesau]

ceres-solver/ceres-solver#1115

[afabri]

ok, so they say that they only support the latest public release of Eigen. Fair enough. That means that we should test a CGAL package that uses Ceres onlut on platforms where we have that version of Eigen.