Implementing read and update in the model

Our entire design up to this point has been centered around a form that only appends data to a file; adding read and update capabilities is a fundamental change that will touch nearly every portion of the application.

It may seem like a daunting task, but by taking it one component at a time, we'll see that the changes are not so overwhelming.

The first thing we should do is update our documentation. Open the abq_data_entry_spec.rst file in the docs folder, and let's start with the Requirements section:

Functional Requirements:
  * Provide a UI for reading, updating, and appending 
    data to the CSV file
  * ...

And, of course, we should also update the part that is not required, like so:

The program does not need to:
  * Allow deletion of data.

Now, it's a simple matter of making the code match with the documentation. Let's get started!

Adding read and update to the CSVModel class

Take...