Update TileEntitySolarPanel.java

Halving the output of the solar panel, if I'm not mistaken. They've gotten wayyyy cheaper and should be a bit nerfed in power output.

So at lightvalue 15 it's 15/15 = 1 which translates to 1MJ/t, aye? Well, with 15/30 you'll get 0.5 MJ/t, then. Fairly reasonable in my opinion.

I might be wrong.