Move output of dhdl file entirely to edr, rather than having a separate text dhdl file
Information stored for computing free energy quantities (such as dh/dl, energy differences between different lambda states) is already stored in the .edr file, where the .dhdl.xvg file can be recreated by running gmx energy -dhdl. Given this capability exists, it would be better to remove the separate printing of dhdl files at runtime.
This is not a high priority, but should be done eventually. Since the current dhdl file works quite well, it will be important to validate that the .dhdl produced by .edr under a wide range conditions is indeed correct before removing existing working functionality. This was checked fairly well at one point, but it's not clear what differences have creeped in since then. For example, we might need the dhdl information at a different frequency than the other edr information, so separate nst's need to be supported