Clean and fix

[rebeccanash]

• General code cleaning and edits
• Properly document all functions and all data files
• Add data processing functions e.g. data_forest_plots(), data_param_table(), data_outbreak_table()
• Edits to data files to fix non-ASCII error
• Fix for R CMD Check "no visible binding for global variable" (not sure if this is a great way of dealing with it, but works for now)
• Change load_data() to load_epidata() to avoid conflict with devtools function of the same name
• Moved data files from data folder to extdata folder
• Update licence to GPL (>=3)
• Fix R CMD check badge
• Add epireview hex sticker