Migrate all project dependencies to Pipfile
    
    This commit removes the multiple requirements.txt files used to install
    and manage dependencies and opts in favor of the Pipfile managed by
    Pipenv:
    
        https://github.com/pypa/pipfile
        https://pipenv.org/
    
    The purpose of this change is to maintain DRY principles. This makes our
    dependencies easier to manage by managing them in one place and only
    dividing them by standard dependencies or development dependencies.
    
    Future changes after this commit will include documentation improvements
    and updates to the Dockerfile to install dependencies in the Pipfile
    format.