F tables converter

Summary

Two new converters:

• XLSXConverter (using pd.read_excel)
• CSVConverter (using pd.read_csv)

... both responsible for converting tables to structure elements.

Both table converters use existing structure elements:

• The rows of the table become DictDictElements
• The columns can then be crawled using the created Dict*Elements

Both converters accept a lot of options as defined by the pandas library. All options supported by the converters are explicitly enabled using the get_options function.

Focus

• Please check whether the enabled options are reasonable. Do we need more?
• Are there enough unit tests for the majority of cases?
• Does the reuse of DictDictElements make sense or are there important limitations?

Test Environment

There are unit tests provided in this MR.

Check List for the Author

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.