add images for DUO icons

[mcourtot]

using foaf:depiction for rendering in OLS

(see also https://github.com/EBISPOT/OLS/issues/419)

from Chisato: we have decided to share the pictograms under the CC-BY-4.0 license and GA4GH copyright.

Also, we revised and included three additional images to cover all of 8 DUO codes in Machine-Readable Consent Guidance.

Please refer below for out latest pictograms;

• DUO pictogram 2.3 https://drive.google.com/file/d/1Sku9Di0Cjs_eWXFeRDN9uO5YDCXSzQkD/view?ths=true

[mcourtot]

Pictograms added to documentation https://github.com/EBISPOT/DUO#duo-pictograms

Still need to add individual annotation properties to DUO terms