Use better names for enums when reporting parameterised tests

[ianthomas23]

Use better names for enums when reporting parameterised tests via a pytest_make_parametrize_id function in conftest.py.

Example before:

$ pytest -v tests/test_dechunk.py::test_dechunk_lines
tests/test_dechunk.py::test_dechunk_lines[0-line_type0]
tests/test_dechunk.py::test_dechunk_lines[0-line_type1]
tests/test_dechunk.py::test_dechunk_lines[0-line_type2]
tests/test_dechunk.py::test_dechunk_lines[0-line_type3]
tests/test_dechunk.py::test_dechunk_lines[2-line_type0]
tests/test_dechunk.py::test_dechunk_lines[2-line_type1]
tests/test_dechunk.py::test_dechunk_lines[2-line_type2]
tests/test_dechunk.py::test_dechunk_lines[2-line_type3]

and after:

$ pytest -v tests/test_dechunk.py::test_dechunk_lines
tests/test_dechunk.py::test_dechunk_lines[0-Separate]
tests/test_dechunk.py::test_dechunk_lines[0-SeparateCode]
tests/test_dechunk.py::test_dechunk_lines[0-ChunkCombinedCode]
tests/test_dechunk.py::test_dechunk_lines[0-ChunkCombinedOffset]
tests/test_dechunk.py::test_dechunk_lines[2-Separate]
tests/test_dechunk.py::test_dechunk_lines[2-SeparateCode]
tests/test_dechunk.py::test_dechunk_lines[2-ChunkCombinedCode]
tests/test_dechunk.py::test_dechunk_lines[2-ChunkCombinedOffset]