Revert dense matrix (quantumlib#804)

- Oops, I got too aggressive changing matrices to dense.
- These two were not sparse matrices.

docs/fqe/tutorials/fermi_hubbard.ipynb

@@ -383,7 +383,7 @@
    "source": [
     "from fqe.algorithm.low_rank import evolve_fqe_givens\n",
     "\n",
-    "umat = expm(-1j * e_time * hopping_matrix.todense())\n",
+    "umat = expm(-1j * e_time * hopping_matrix)\n",
     "evolved_wfn = evolve_fqe_givens(init_wfn, umat)"
    ]
   },
@@ -530,7 +530,7 @@
     "trotter_steps = 10\n",
     "\n",
     "# Evolution under the hopping Hamiltonian for a single Trotter step.\n",
-    "umat = expm(-1j * hopping_matrix.todense() * (e_time / trotter_steps))\n",
+    "umat = expm(-1j * hopping_matrix * (e_time / trotter_steps))\n",
     "\n",
     "# Simulate each Trotter step.\n",
     "current_wfn = copy.deepcopy(init_wfn)\n",