Add support and documentation for the functions supporting combined data analysis

[ppreshant]

There's plenty of new general functions .. till 21-..R and not enough documentation.

This will help me pickup the analysis after a long gap too!

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1. Use 21-merge_cytosets_and_save.R to combine data using a regex.
2. Run with the combined folder in the regular analyze_fcs.R script

[ppreshant]

Could also include the combined data workflow into analyze_fcs.R

get user input directory regex and combined dir name and a switch