Reduce-T and rescale-T option now set thermostat based on target temperature

This resolves #127 (closed). The target temperature is now updated using the previous (target) temperature of the thermostat. This avoids the weird behavior of reducing based on the instantaneous temperature. In addition, it is now allowed for delta_T to be positive or negative, so that the option reduce_T can be used to increase or reduce the temperature. I would suggest allowing for additional names (as in augment_T etc) to be allowed, but this should be a separate issue, and a separate commit. Documentation (PW_INPUT.def) updated in this commit as well.