Add LEFT_NUTSX and RIGHT_NUTSX to Nuts2Json

[joewdavies]

Currently in the GISCO.NUTS_BN_XX_20XX files we have the NUTS IDs to which the nutsbn belong to, but they are not included in nuts2json.

In the attached IMAGE you can see that eurostat-map currently give the nutsrg a stroke instead of the nutsbn. This is because only the nutsrg currently has an identifier that can be matched to the statistical data NUTS codes.

The use case for this is IMAGE: which uses 'mixed' NUTS levels e.g. where gaps in the dataset are filled with different NUTS levels. At the moment, eurostat-map simply adds all NUTS regions (1,2,3) to the map then hides/shows them according to whether they are present in the dataset or not.