Refactor/clean-up parts of the code

[kingsleyadam]

This runs a refactor/clean-up on a few parts of the code.

• Update get_device_by_class to get_devices_by_class to be consistent with other functions.
• Add new get_devices() public function to allow any external code to fetch the list of devices. Use this in tests.
• Move _config, _devices to init of the FreeAtHome class.
• Move the _function_to_device_mappingto a newconst.py` file to clean-up and reduce the size of the FreeAtHome class.
• Use a dictionary to map Function to Base in the new const.py file instead of a list. This enforces a function can only be defined once, but a device class can be assigned to multiple functions.

[kingsleyadam]

Fyi @derjoerg on the changes to Function to Class mapping for any future functions.

[derjoerg]

@kingsleyadam
From a first look I really like the simple and clean approach.