Do we add Eigen-specific functions?

[GPMueller]

Since we need modern compilers anyways and Eigen is quite small and header-only, I don't see a particular reason not to.
Examples would be:

• dot which could take two vector fields and output a scalar field
• cross
• mean which would give a mean vector instead of a scalar

@NOhs does the usage of Eigen (e.g. std::vector<Eigen::Vector3f> correspond to your use-case?

Note the mean function could not be applied by using the templated mean, as it requires atomics. Thus it would have to be specialized.