add validation check to make sure x_spi_platform_code is unique across all datasets

[jen-thomas]

x_spi_platform_code should be the name of the platform, the name of the project and the primary DOI (not the DOI of each version).

For example: UBXH3_ace_3973199

There are currently no checks to make sure that this is unique across all datasets so there could be the problem that the person creating the dataset forgets to change this and then it would not be unique.

Emodnet needs this to be unique as it is used as a primary key. It is also displayed on the landing page of the dataset on Emodnet physics.