Added functions to specify color array for each point - this will allow…

[vimsh]

… to have a gradient coloring from low to high points (see main function in GridPlot3D.java as other plots' samples just use random colors rather than gradient)

I had used your lib in my little project and had made these changes for more colorful plots, so perhaps it'll be useful for others.

image: https://camo.githubusercontent.com/817cd21de1cb488c76d2809624e01e7a5c2b95ef/687474703a2f2f693834362e70686f746f6275636b65742e636f6d2f616c62756d732f616232332f766c6164696d73682f6d61667363616c696e675f686f77746f2f76655f33642e706e67