feat: KEGG pathway annotation

[edkerk]

Description of the issue:

With single subSystem assignments (#11), that do no necessarily match KEGG pathways (#307), and multiple KEGG pathways that can match one reaction, there should be dedicated KEGG pathway annotations. These can be stored in the rxnMiriams field (use RAVEN's editMiriam function), as KEGG pathways is on identifiers.org.

Note that some annotation is already available here, but the information in that file is incomplete: this information should be freshly taken from KEGG. If this would yield additional kegg.reaction annotations, then these should of course also be added to the model.

I hereby confirm that I have:

• [x] Tested my code with all requirements for running the model
• [x] Done this analysis in the main branch of the repository
• [x] Checked that a similar issue does not exist already
• [ ] If needed, asked first in the Gitter chat room about the issue