Use SPDX license Identifiers

[rettinghaus]

Currently the CMIF-Creator uses strings for the license that are kinda hard to process, the only thing you can trust is the url. I would suggest to use a SPDX Identifier, so instead of "CC0" use "CC0-1.0", and "CC-BY-4.0" over "CC-BY 4.0".

Maybe then even change the url to use the SPDX resolver?

[rettinghaus]

Here are some links to relevant open data licenses:

• https://opendefinition.org/licenses/
• https://resources.data.gov/open-licenses/