Integrate vocab for extraction of platform information

[ferrighi]

Handling of platform information can be improved through the use of vocab.met.no. Additionally, the current implementation does not handle multiple platform properly. I suggest to support for now the following options:

• platforms: short-name
• platforms: short-name1, short-name2
• platforms: some long name (short-name1)
• platforms: some long name (short-name1), some long name2 (short-name2)

Using vocab we can populate directly, short_name, long_name and resource in MMD.

This will require a modification of the vocab function as well as nc_to_mmd.

[ferrighi]

I will also add extraction of instruments through vocab, since it is related. For now I will not check that the instrument is indeed registered as on board the platform. This feature can be added in the future.