Fix tests

[awhite862]

• Add tests for new Taylor evolution algorithm
• Update the reference wavefunction data in tests, so that the expansion is converged for dense operator evolution
• Additional tests (restore 100% line coverage)
• Properly treat the case of an empty FermionOperator/SparseHamiltonian (+test)

[klgunst]

All tests pass.

Some warnings:

../../../../Codes/anaconda3/envs/fqe/lib/python3.9/site-packages/openfermion/hamiltonians/hartree_fock.py:11
  /home/kgunst/Codes/anaconda3/envs/fqe/lib/python3.9/site-packages/openfermion/hamiltonians/hartree_fock.py:11: DeprecationWarning: Please use `OptimizeResult` from the `scipy.optimize` namespace, the `scipy.optimize.optimize` namespace is deprecated.
    from scipy.optimize.optimize import OptimizeResult

tests/davidson_test.py: 119 warnings
  /home/kgunst/qsimulate/software/devel/OpenFermion-FQE/src/fqe/algorithm/davidson.py:69: DeprecationWarning: `np.complex` is a deprecated alias for the builtin `complex`. To silence this warning, use `complex` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.complex128` here.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    if isinstance(val, (float, complex, np.complex128, np.complex)):

tests/low_rank_test.py::test_fqe_givens
tests/low_rank_test.py::test_charge_charge_evolution
tests/low_rank_test.py::test_double_factorization_trotter
  /home/kgunst/Codes/anaconda3/envs/fqe/lib/python3.9/site-packages/scipy/sparse/_index.py:103: SparseEfficiencyWarning: Changing the sparsity structure of a csc_matrix is expensive. lil_matrix is more efficient.
    self._set_intXint(row, col, x.flat[0])

Code coverage shows a few branches never been taken: pytest --cov=fqe --cov-report=html:coverage --cov-branch.

coverage.zip