Added ggplot2 functions, contour lines

New functions that use ggplot2 were added: gcf, gcf_grid, gcf_data, gcf_func. These can also be used in the normal functions using gg=TRUE.

Contour lines can be added to plots using with_lines=TRUE, and only lines with no fill can be made using lines_only=TRUE.

Accepted to CRAN on 5/20/19.