Config file: mechanism for keys names changing
In the nabu config file, some keys are likely to change (or be added/removed) in the future.
There is already a nabu_config_version in the [about] section.
Now we need a mechanism to access one given key with a future-proof way.
For example, if paganin_delta_beta is changed to delta_beta:
get_property(phase.DELTA_BETA) --> DELTA_BETA: {"2020.1": "paganin_delta_beta", "2020.2": "delta_beta"}