External EN bonds itp

[csbrasnett]

Add a flag, -ew, to write out bonds for elastic networks to an external en_bonds.itp file, in the style of Go or Olive bonds.

This has been motivated by some work on visualising cg molecules at martini_vis for now. If the bonds are written out externally, then they can be used (after some editing elsewhere) to visualise the elastic network separately to the bonds inherent in the molecule. Maybe this is useful, maybe it isn't, it's a feature at least!

pros:

• might be able to visualise elastic networks better

cons:

• relies on users actually writing #include "en_bonds.itp" in their [ bonds ] directive in their molecule's itp file