For the target compound loon plot, determines location in ggmatrix
g_getLocations(target)
# S3 method for default
g_getLocations(target)
# S3 method for l_pairs
g_getLocations(target)the (compound) loon plot whose locations are needed to lay out.
a list of an appropriate subset of the named location arguments
`c("ncol", "nrow", "layout_matrix", "heights", "widths")`. layout_matrix
is an nrow by ncol matrix whose entries identify the location
of each plot in g_getPlots() by their index.