Inverted Color Palettes

Wasn’t so hard once I figured out what was going on in TColor.

Would this commit be appreciated as a pull request?

A short demonstration:

{
   gStyle->SetPalette(kCherry);
   TColor::InvertPalette();

   TF2 *f = new TF2("xygaus", "xygaus", -2, 2, -2, 2);
   f->SetParameters(1,0,1,0,1);

   TH2D* h = new TH2D("h", "H", 50, -2, 2, 50, -2, 2);
   h->FillRandom("xygaus", 50000);
   h->Draw("COLZ");
}