Step parameters that accept path strings should accept pathlib.Path objects

[jdavies-st]

Currently, Step parameter validation requires file paths to be strings, specifically for overrid_<reffile> cases. pathlib.Path objects should also pass validation. Specifically this means modifying the custom validation functions here

stpipe/src/stpipe/config_parser.py

Lines 331 to 334 in b928337

	validator.functions["input_file"] = _get_input_file_check(root_dir)
	validator.functions["output_file"] = _get_output_file_check(root_dir)
	validator.functions["is_datamodel"] = _is_datamodel
	validator.functions["is_string_or_datamodel"] = _is_string_or_datamodel