Change function module name

[negillett]

Currently there is only one lambda function, in lambda_functions.py.
Based on https://docs.aws.amazon.com/lambda/latest/dg/lambda-python-how-to-create-deployment-package.html, it doesn't seem possible to store multiple lambda functions in a single Python module.
Each lambda function should therefore live in its own, uniquely named module.

Change the name of the existing lambda function and test modules to set the correct precedent for naming.