Change to pydantic model based validation of the system config file

AIM

System config validation changed to pydantic based model validation. A new exception class created to simplify the error messages displayed.

AUXILLARY CHANGES

Associated update to unit tests.

DETAILS

• All pydantic based models for system configuration included to file src/swatch/stubs.py .
• Corresponding changes made to src/swatch/config.py to allow pydantic based validation of the system config yaml input file.
• Requirements for pydantic model enabled in requirements.txt .
• New exception ConfigValidationError defined in src/swatch/exceptions.py which handles the ValidationError generated by pydantic for simplified user readable error message.
• Minimal updates to tests/config/test_parse_parameter_file__.py and tests/config/test_parse_system_file__.py such that the new tests are compatible with incoming changes.
• Functions expand_link_id() and parse_index_list_string() changed to corresponding internal functions _expand_link_id() and _parse_index_list_string() . The test function for these were subsequently changed to test__<FUNCTION_NAME>__() to create a new nomenclature for test functions for internal functions.

CORRESPONDING CODE CHANGES

Requires !116 (merged) . Fixes #40 .