Adds v_sig calculation to force loop in ANARCHY-DU

This adds a max() in the force loop for the signal velocity, for ANARCHY-DU. This should hopefully help with some time integration problems we've been seeing, but should be considered a quick fix (note the comment) so that people can get on with XL related stuff.

MS: are you happy with this to go into the codebase? I don't like that it's masking an issue but I guess we're kinda up against it :)