Y Axis Combination Matrix

I am sure that this is something someone will want, but I currently don't have the capacity to implement this.

But, this would a great issue for somebody to try and make a pull request. Thus, if you want to learn more about the internals of ggplot, writing an extension and contribute to an open source, just comment with any questions you have and I will try to help you as good as I can.

A good place to start is to look at the [`render_axis_h()`](https://github.com/const-ae/ggupset/blob/master/R/axis_combmatrix.R#L110) and the [`make_combination_matrix_plot()`](https://github.com/const-ae/ggupset/blob/master/R/axis_combmatrix.R#L228) function in [axis_combmatrix.R](https://github.com/const-ae/ggupset/blob/master/R/axis_combmatrix.R) file.

In addition it probably is also a good idea to use the debugger to understand how ggplot2 step by step builds up the plot, by calling `debugonce(ggplot2:::ggplot_build.ggplot)`.