DenseBase Concatenation

Describe the feature you would like to be implemented:

This proposal contains a meta-template implementation of DenseBase concatenation supporting arbitrary DenseBase expressions. There is a C++14 version to fit the next Eigen release and a cleaner C++17 version.

Would such a feature be useful for other users? Why?

It's a pretty classical thing to do 😄

Implementation of the requested feature:

https://godbolt.org/z/fdafe9c5z

Edited by Stéphane T.