refactoring eln base sections

@josma @hnaesstroem and myself are about to propose an update of nomad.datamodel.metainfo.eln.__init__.py base classes.

The main points we have identified so far are:

• ElnBaseSection must be dismembered in the new most general base section we have available (Entity, Activity and similar)
• ElnActivityBaseSection must be harmonized with the Activity, and become the same object
• Substance. We decided last week to split Substance in "intrinsic" properties like CAS_number or formula and "management" properties like supplier, batch_number, buying_date, purity(depends on the batch of our substance), etc. Given this antecedent, we would like to fix the intrinsic properties at least as a first step, so we can start to use this class coherently wherever in schemes in Nomad
• SampleID should change his name in EntityID or ID so we can use it in a more general way in our entries. It should also get harmonized with existing lab_id
• ElnWithFormulaBaseSection

Other identified changes will be listed in checkboxes in the next comments to this issue.

We will ask better to @mscheidg how to behave with changes that would affect the results section, like the lab_id.