Cleanup on Aisle "Model"

Made some very small changes that impact very little, but represent
things that I've learned by working with Scala for the past week or so.
Most identifiable amongst the changes is the removal of duplicate
vals/parameters in the model classes. It was a rookie mistake, likely
caused by me coming from Java.

Signed-off-by: Peter Zastoupil <peterzastoupil@gmail.com>