heat_map() draws a double-clustered heatmap
for path coefficients analysis. Please be cautious
that this function acts only on numeric
variables/columns (see example on dtraw2 data set).
Users for drawing other types of heatmaps may
use heatmap.3, ComplexHeatmap and pheatmap R packages.
Where an example is given in the vignette manual of this
package (Path.Analysis_manual.Rmd)