Tweaks to shield generators

Summary: 
- A few name changes and var cleanups. 
- Boosting or removal of some settings' upper limits. The maximum power draw for shield capacitors is now 5 million watts (generators don't draw power directly from the grid). 
- After taking damage, an energy field will have a temporarily delayed recharge. 
- Seems to be a bug with the circular shield gens that they won't pick up turfs in a radius greater than 20-30. Could probably do a bit more testing, but I'm stumped in the mean time. 

See commit log for more details.