[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]

https://github.com/ceres-solver/ceres-solver/issues/1115