Added the ".holdfixed" option to the solve commands, which should make the GAMS compiler convert fixed variables into parameters before feeding the problem to the solver.

NOTE! Regarding the last change concerning the inclusion of flow units into the suft sets: Now conversion equations are generated for flow units, which is required for EV batteries. However, without a defined "fuel" or input energy, traditional flow units (wind, solar, RoR) stop generating energy due to the seemingly nonexistant energy source. This problem can be circumvented by defining free "fuels" for flow units, but is a rather unnecessarily convoluted way of handling things.