Resolve "refactoring eln base sections"

Closes #1152 (closed) by adding the Substance and Ensemble classes with their superclass System to nomad.datamodel.metainfo.eln. In addition the SampleID class was modified to get automatically filled by data from the metadata.main_author. Merge also includes a fix to the GUI for rendering undefined times. Two tests have been added to test the normalizers of the three added classes. The test coverage of nomad/datamodel/metainfo/eln/init.py is therefore the same 75 % as before the additions.

EDIT: Implemented new methods in the new Formula class and updated the Substance class to use these. Ready for review.