Enlarge dG limits

Hi Jochem,

would be possible to enlarge dG_limits in make_dict function form {"start": 10, "end": 50} to {"start": -10, "end": 50}. I wanted to simulate the calculation of the D-uptake of a peptide, given the parameters of the Linderstrøm-lang model in peptide app. But hard limit is 10-50 KJ/mol. We have initial dGs calculated from previous experiment having negative values. Shall I create pull request? I push the branch for review? Thank you for considering.