Update advvolt.pawn

Reworked, even if RMS calculation always gives positive result, for DC voltages <0 it's now displays with minus sign as normally expected. Additional Screen layout added
Updated and extended

negative Sign now for negatve Voltages, even if RMS result is always
positive Result. One more Screen Layout added
Merge pull request #7 from schmidhorst/patch-1

Update advvolt.pawn