Added ELN Base Sections

Closes #1152 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`. 

The `Susbtance class uses the the new `Formula` class from `nomad.atomutils` and two new methods were introduced to this class.

Merge also includes a fix to the GUI for rendering undefined times.

Changelog: Added