Recommend SPDX as the preferred list of licenses

[jdvorak001]

In line with the current DataCite Metadata Kernel v.4.4, p.28, we should recommend the SPDX URIs, e.g. http://spdx.org/licenses/CC-BY-4.0 . The scheme would then be https://spdx.org/licenses/ .

This should also be demonstrated in examples. One Publication example needs migration.

[ACz-UniBi]

Dear @jdvorak001 ,

to propose the use of the License element in Publications

<License scheme="https://spdx.org/licenses">https://spdx.org/licenses/CC-BY-3.0</License>

I would also suggest an example in the Products for the record of Product id="Products/729487" like

<License scheme="https://spdx.org/licenses">https://spdx.org/licenses/CC0-1.0</License>