updated integrate annotation and report

[MahdiMoradiMarjaneh]

Updates include:

• added Sankey plot (riverplot) feature to the integrate report (#144)
• added proportional barplot feature to the integrate report showing what percentage of each cluster comes from each group variable (#144)
• fixed issue #142. Labels in clustering plots generated by annotate_integrated_sce() now reflect the correct reduction_method used in cluster_sce()
• minor updates to liger_preprocess()

List of file changes:

modified:   R/annotate_integrated_sce.R
modified:   R/liger_preprocess.R
modified:   R/report_integrated_sce.R
new file:   inst/rmarkdown/templates/integrate/skeleton/categorical_covariates_2.Rmd
modified:   inst/rmarkdown/templates/integrate/skeleton/skeleton.Rmd