add uncertainty and comment to quantity

[linuxrider]

needed for echemdb/electrochemistry-data#89

Fixes #80

[DunklesArchipel]

I would also add a unit field. The "number" field should probably be "value", as is the case for "quantity"?

[DunklesArchipel]

To test this, we should add some tests to any of the YAML files in the example section.
https://github.com/echemdb/metadata-schema/blob/main/examples/objects/system.yaml
and maybe the output JSON
https://github.com/echemdb/metadata-schema/blob/main/examples/file_schemas/echemdb_package.json

[DunklesArchipel]

Running local tests with pixi run validate results in the following error

Pixi task (validate-objects): bash -c "    find . -type f -name \"*.json\" -exec sed -i '/$id/d' {} +
    for file in curation eln experimental figure_description projects source system; do
        check-jsonschema --schemafile ./schemas/${file}.json ./examples/objects/${file}.yaml
    done
    "                                                                                                                                                                                                                          
sed: -e expression #1, char 0: no previous regular expression                                                                                                                                                                  
ok -- validation done
ok -- validation done
ok -- validation done
ok -- validation done
ok -- validation done
ok -- validation done
Schema validation errors were encountered.
  ./examples/objects/system.yaml::$.electrolyte.temperature: Additional properties are not allowed ('uncertainty' was unexpected)

I am not exactly sure how the referencing works and, hence, locally the data is tested against the web version of the older metadata schema.

[DunklesArchipel]

I changed pH into a quantity