Show amount over limit on pg/cpu/drone displays

Hi.  My first submission to pyfa, but it's been an annoyance for me.

I screwed up the original comment, it's not remaining units, but units over so it's more obvious how much I have to trim.

It's a pretty small change.  I'm not sure if it would break anything else that might use gauges tho.

--- Commit comment -----
Update powergrid, CPU, drone bandwidth, and drone bay displays to show remaining units so I don't have to do math.   Could affect other gauge displays?