Remove Eigen plugin (was ACTS-478)

Original author Paul Gessinger-Befurt @pagessin

We build with Eigen, and use our Eigen plugin. This is copy of ATLAS' Eigen plugin.

When we build in Athena, this works, because the plugin is identical, meaning all Eigen types remain compatible. This might not be the case if the plugins are not identical.

We need to investigate the impact of this. It might be necessary to build against the experiment specific Eigen plugin, as soon as we don't have an identical plugin. This experiment specific plugin needs to include all of our plugin code. To limit the danger here, I think we should at least make our Eigen plugin as small as possible.

I'll add an MR that reduces the plugin to perp, perp2, phi, theta and eta methods. We will have to adjust the Acts build in the atlas externals though. Maybe we could ship with an export of ATLAS' Eigen plugin and allow enabling that via cmake option?

<asalzbur> <rlangenb>

 

In essence, we're removing the Eigen plugin and use helper functions to retain functionality.