`DataSource` refactoring: adding the `utils` directory and custom exceptions.

[RicardoJSandoval]

In this pull request I'm adding the utility functions that are the building blocks for the rest of the DataSource refactoring. This pull request contains the following files:

• exceptions.py: contains custom exceptions
• utils/load_utils.py: contains a utility function to extract the contents from a zip file
• utils/download_utils.py: contains utility functions to download files from specified URLs (we also have the capability to do this asynchronously via the concurrent.futures library) and determine which files are not found locally and, thus, need to be downloaded.

This is the first pull request from the series of pull requests I specified here.

[RicardoJSandoval]

I'm closing this request and going to open a new one, as I accidentally included commits that I shouldn't have.