extract atoms doesn't work for custom properties

[Joroks]

With the fix property/atom lammps command it's possible to create custom per atom properties that properly migrate across processors. Currently, our extract_atom wrapper only supports this with vector data but not with matrix data. This is due to a bug in lammps and a missing API function to query the size of the matrix.

I've already opened upstream issues for this and a fix is currently in progress (lammps/lammps#4304).

This new feature allows us to query the size of any property though the API. This way, we don't have to hardcode these sizes anymore, making our wrapper more robust. It also reports wheter the property has ghost atom entries.

I'd suggest that we move to a version that implements this as soon as it releases (possibly even as a preview or rc) and adjust our extract_atom wrapper accordingly. gather and scatter! should be adjusted as well.