v2.1: support for visualising subsets of cells

Minor update to v2.1 to support the visualization of subsets of cells

Updates include:

• Ability to visualize subsets of cells
  --- Users can now subset cells by any categorical metadata e.g. visualize only certain libraries or clusters
  --- For dimension reduction plots, cells that are not selected will still be displayed as very light grey in the background
  --- For violin/boxplots & proportion plots & bubbleplot/heatmaps, users can also subset cells
  --- Added "Select all" and "Deselect all" buttons for user convenience
  --- Users can also choose to disable this function by specifying this new argumentenableSubset = FALSE in the makeShinyApp or makeShinyCodesMulti function (related to issue #15)
  

• Bugfix and updates to improve user experience
  --- Fix bug where short gene names cannot be searched (related to issue #19)
  --- Add an argument defPtSiz = 1.25 in the makeShinyApp and makeShinyCodesMulti function to specify default point sizes (related to issue #18)
  --- Updates to support the latest version of SCE (related to #11 #12 #17)