getBranch now allows php 'falsey' keys (i.e .0)

Refactoring ParameterTree to improve architecture and tests

Added ValueExists exception and updated set behaviour to throw exceptions when trying to override an existing key accidentally.
Completed test coverage for getBranch method

Fix for losing empty arrays

allow getBranch to return null values

Update to allow getBranch to return null values if the value fetched exists but is set to null

Initial import + tests