Suppress high_level_api import warning

See merge request !133

Add more XLSX tests

See merge request !131

- Added parameter use_id_for_identification to JsonSchemaExporter, which sets foreign key to id and adds an 'id' column to all tables
- Added parameter return_data_schema to merge_schemas. If set, merge_schemas returns a data_schema as well as the normal model_schema

- Added new file export_import_xlsx. In addition to private functions for generating templates, schemas, and data, there is a new public function export_container_to_xlsx which generates an xlsx file containing the data from the given records at a given path

- Changed a print warning in fill_xlsx to warnings.warn for easier filtering

TST: Use _validate_jsonschema instead of jsonschema.validate in fill_xlsx, move _validate_jsonschema to own file and and adjust tests to match

Enable validation in table_json_conversion.convert.to_dict for use in XLSX-converter

See merge request !129

See merge request !128

Remove support for Python 3.8

See merge request !127

Fix pylint errors

See merge request !126

XLSXConverter paths validation

See merge request !125