Implicit indices need to come in normal and conjugate forms.

Right now, two objects with the same implicit index sitting next to each other are assumed to be connected. However, for e.g. \psi \bar{\chi} with \bar a DiracBar, this is not the case, and the lines go in a different way. In order to determine this, we need to have the concept of indices and conjugate indices.

The can_swap routine currently has an exception for DiracBar objects, but this needs to be generalised.