Normalization of years of publication to EDTF format

[tijmenbaarda]

Necessary to make sorting by year possible in the VRE. It will not be possible to cover all possible formats, but at least the most basic ones (simple year and simple range) should be covered.

Python package for EDTF: https://pypi.org/project/edtf/

[tijmenbaarda]

Since Jeroen is okay with the current situation (non-normalized years), we don't have to prioritise this for now.