Transfer frequency model from old project

[YifanWang0]

From the old Gender Analysis Toolkit project, we will be transferring over the frequency module. Breaking it down into a sequence of steps, we will:

• Transfer over _run_analysis and the related helper functions
• Transfer over different functions for running the analysis by different categories (by_date, by_document, by_gender, by_identifier, by_metadata) and their related helper functions
• Update the analysis module to incorporate the Corpus model once it is in place
• Build a frontend interface for users to interact with the module in document page