Metadata column names in constants

[Nelly-Barret]

For now, I often do things like this:

self.metadata["JSON_values"] ...
self.metadata["name"] ....

This is not very good because:

1. I didn't check beforehand whether the column exists in the data frame (thus it will raise an exception if it does not exist)
2. A small typo in the column name would raise an exception, and in any case, it is always better to define such repetitive strings in constants.

[Nelly-Barret]

Done through https://github.com/Nelly-Barret/BETTER-fairificator/pull/82