Added tests for TimeFrame class. Cleaned up.

src/topupopt/data/finance/utils.py

src/topupopt/problems/esipp/problem.py

src/topupopt/problems/esipp/time.py

src/topupopt/problems/esipp/utils.py

tests/test_esipp_problem.py

tests/test_esipp_time.py

tests/test_esipp_utils.py