Merge branch 'cleanup/elementpointer' into 'master'

Cleanup/elementpointer

Removes all occurences of ```EntityPointer``` (except multidomain).

Removes deprecation warnings related to ```EntityPointer```s.

All methods returning ```EntityPointer``` now return Entities.

```InteractionVolume```s in decoupled store ```ElementSeed```s.

```CCElementGeometry``` stores ```Element```s.

See merge request !4