Refactor ELN Base Sections

Issue

Description

Currently, general base sections are located in the eln module and need to be moved to separate module.

These sections include:

• Entity
  • System
    • Ensemble
    • Substance
  • Instrument
• Activity
  • Process
  • Measurement

The plan is to create a new module basesections.py in nomad.datamodel.metainfo.

Tasks

• Copy the above mentioned base sections
• Find way to notify users about deprecated sections