Complete the set of unit tests for the config.py python script.

• Complete the set of unit tests for src/swatch/config.py. Following up on !110 (merged).
  • The following iteration includes unit tests for LinkModeManager class and expand_link_id(), parse_index_list_string() and parse_system_file() functions.
• As an auxiliary task, developed test suite for src/swatch/io_settings.py at tests/io_settings.
• pytest and check packages have been used to develop the unit tests.
• Updated exceptions from RuntimeError to InvalidConfigFormatError for all instances.
• Raises issue #51 .

The following set of general rules were followed while developing the unit tests.

• The test suite for any particular script is developed in a dedicated directory under tests/<PY_FILE_NAME>/ . The corresponding data used by the tests are stored in tests/<PY_FILE_NAME>/references/.
  • In this case these directories are tests/config/ and tests/config/references .
• Test scripts for classes are named as test_CLASS_NAME.py while test scripts for non-internal functions are named as test_FUNCTION_NAME__.py.
  • The internal functions do not have dedicated test scripts for them but they are unit tested through the functions or classes that call them.