For the target compound loon plot, creates the final grob from the class of the `target` and the `arrangeGrob.args`
l_createCompoundGrob(target, arrangeGrob.args)
a grob (or list of grobs) that can be handed to `gTree()` as `children = gList(returnedValue)` as the final grob constructed for the compound loon plot. Default for an `l_compound` is to simply execute `gridExtra::arrangeGrob(arrangeGrob.args)`.
the (compound) loon plot
arguments as described by `gridExtra::arrangeGrob()`