This function should not be used directly,
only inside .plot_boxgraph_RNAratio_TCGA()function.
.RNAratio_selection(df, gene_ratio)output dataset generated from .RNAratio_calculation() function
gene ratio name
input argument for selected variables generated
from .plot_boxgraph_RNAratio_TCGA() function
Other helper function for differential expression analysis: 
.RNAratio_calculation(),
.RNAratio_tumortype(),
.RNAseq_all_gene(),
.RNAseq_ind_gene(),
.RNAseq_tumortype()