Minor fix to stay on CRAN

Minor fix to stay on CRAN.

Fixed packages in Suggests to either be Depends or non-essential.

Accepted to CRAN on 9/10/24.

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.

First release

R package ContourFunctions accepted to CRAN on 5/4/17.