Include NCIT description in unii uploader

[newgene]

UNII data source includes a mapping to NCIT id in unii.ncit field.

In this translator tool, we populated unii.ncit_description field based on our NCIT API at https://biothings.ncats.io/ncit on the fly, e.g.:

https://biothings.ncats.io/annotator/CHEBI:5790?fields=unii.ncit

It would be better if we could include this description field directly in MyChem.info.

[newgene]

The relevant code we populate this description field on the fly is here:

https://github.com/biothings/pending.api/blob/ac6034f52475b7e3d99a2c0fce0631de9526513a/web/handlers/annotator.py#L68-L102

which we can use in UNII parser as well.

[newgene]

also less likely, but we can confirm UNII data source does not include NCIT description first.

[DylanWelzel]

added unii.ncit_description in #172