Virtual Sites Relative can dangle

[fweik]

If the reference particle of a virtual site is removed, and virtual sites relative is enabled, this leaves the system in an invalid state, and it will crash on next propagation. I think the straight forward solution for this turning virtual sites relative into a bond, at least internally. There is already a mechanism in place that removes dangling bonds on particle removal. This would also make this more consistent with e.g. rattle + we would get per particle virtual sites implementations out of the box. @RudolfWeeber any thoughts on this?