add chapter, check_dataset_functions

[ehwenk]

• added new chapter toward [traits.build adding studies functions] Better options for checking data quality traits.build#137
• these functions will also become traits.build functions, but still should be included here , as users might want to change their exact formulations
• still need to add the duplicates_across_datasets function in the future

[yangsophieee]

Such good functions Lizzy! I wish I had these back when I was entering a lot of datasets!