referencing a "definitions" + "data" file into another "data" file

Following the work started in #954, to complete our reference requirements we would like to have the possibility to reference into a ReferenceEditQuantity of our schema other files that may have different structures.

The last line of the following schema restricts to a list of files with the "Substance" structure.

Actually, we have cases in which we may want to reference into this composition more than one chemical compound, each of them belonging to Substance class or to other kind of classes.

definitions:
  name: 'Electronic Lab Notebook schema fot MOVPE-STO'
  sections:
    Substance:
      base_section: nomad.datamodel.data.EntryData
      quantities:
        comment:
          type: str
          m_annotations:
            eln:
              component: StringEditQuantity
        chemical_name:
          type: str
          description: The name of the chemical that is typically used in literature
          m_annotations:
            eln:
              component: StringEditQuantity
    MovpeSto_schema: 
      base_section: nomad.datamodel.data.EntryData
      m_annotations:
        eln:
          dict()
      sub_sections:
        materials:
          repeats: true
          section:
            m_annotations:
              eln:
                dict()
            sub_sections:
              substrate:
                repeats: true
                section: 
                  base_sections:
                    - nomad.datamodel.data.EntryData
                  m_annotations:
                    eln:
                      dict()
                  sub_sections:
                    composition:
                      repeats: true
                      section: Substance  # I would like not to have only Substance here

data:  
  m_def: ../upload/raw/movpe_sto.schema.archive.yaml#MovpeSto_schema 
  materials:
    substrate:
      composition:
        - "../upload/raw/Copper_II_Selenide.archive.json#data"  # this file must be compliant solely to Substance class
        - "../upload/raw/Zinc_Selenide.archive.yaml#data" # this file must be compliant solely to Substance class

Relaxing the "section: Substance" restriction we have several ways the user can act. The user can:

• decide to leave this Substance class in the "sections" of his master schema file together to other needed classes and simply pick one and put it in the m_def line of his "data"

• reference to a file having its own schema definition:

definitions:
  name: 'Substance schema'
  sections:
    Substance:
      quantities:
        comment:
          type: str
          m_annotations:
            eln:
              component: StringEditQuantity
        chemical_name:
          type: str
          m_annotations:
            eln:
              component: StringEditQuantity
data:
  m_def: Substance
  comment: "my chemical description"
  chemical_name: "my chemical name"

• in a more clean way, split the schema portion avoiding to repeat the schema in each of the files to reference (feature already in place) and also not pollute the schema of a specific experiment with Substance class that is more general and used everywhere (at some point these classes shall be elevated to base classes in Nomad)

definitions:
  name: 'Substance schema'
  sections:
    Substance:
      quantities:
        comment:
          type: str
          m_annotations:
            eln:
              component: StringEditQuantity
        chemical_name:
          type: str
          m_annotations:
            eln:
              component: StringEditQuantity

data:
  m_def: ../upload/raw/Substance.archive.yaml#Substance
  comment: "my chemical description"
  chemical_name: "my chemical name"

---

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A further - nice - implementation could be:

section: Substance, Sample, WhatNot

So when creating a new reference the user has a drop down menu to choose which class the file he wants to create is referencing to (I guess this may be again a separate issue)