EZ isomerism and chirality

[fgrunewald]

Implementation for storing chirality and E/Z isomers across double bonds.

Chirallity is simply annotated onto the chiral center with the ordering of nodes giving the improper dihedral defining that center. Note that in order to get R/S classification one still needs to correctly interpret the chiral center. For building coordinates it should be enough to take the imporper dihedral angle defined by the atoms and give it a positive value.

E/Z isomers are annotated to the outermost atoms, which define the isomerism. The annotation consists of the 4 atoms that would form a proper dihedral angle whose value is either cis=0 degrees or trans=180 degrees.

[fgrunewald]

@pckroon the current state is that I think it correctly interprets the chirality but there is no way I can figure out how to get an absolute assignment from this, which means it is nice but also useless