add logic for single string input in get()

[roussel-ryan]

This pull request enhances the flexibility of the LUMEModel interface by allowing the get method to accept either a list of variable names or a single variable name as a string. It also updates the corresponding test suite to ensure this new functionality works as intended.

Improvements to the LUMEModel interface:

• Updated the get method in LUMEModel to accept either a list of variable names or a single variable name as a string, and to return either a dictionary of values or a single value accordingly.

Testing enhancements:

• Added a new test to verify that the get method correctly handles a single variable name provided as a string.