add unambig-ti subcommand

[rvhonorato]

This is a stricter variation of the ti subcommand - for each residue (chain_i) in the interface, it needs to find the closest residue (chain_n) and get the distances between it's CAs. example:

assign ( resid 10 and segid A and atom CA) ( resid 20 and segid B and atom CA ) 42.00 5.00 0.00

It should take as parameters the cutoff to define the interface and a lower/upper boundary for the restraints