Some tests fail with SymPy 1.5

[chenpeizhi]

Two tests in nuclear_test.py and one in spin_one_half_test.py fail due to updates in the latest version of SymPy.

[chenpeizhi]

Need to check the release note of SymPy 1.5. This will be fixed in #22.

[chenpeizhi]

Fixed by d6a1e45, a82d78e, 58f8134, fe59399, and d9f2cb7 in #22.