Add unit support for percentages

Related to https://github.com/nomad-coe/nomad/issues/100. Starting from 0.21, Pint has built-in support for percentages. Unfortunately, also starting from that exact version, this Pint issue is preventing an easy migration as it breaks our parser code.

Tasks:

• Upgrade Pint, once multiplication issue is solved.
• Ensure that new unit definitions get migrated to the front-end.
• Update JS unit code to support percentages.
• Add tests for the JS unit code.