Added (Majorana) fermions to basic operator types.

[brad-lackey]

Added classes Fermion, FermionString, Majorana, MajoranaString analogous to Pauli and PauliString.

• Uses internal coding similar to Paulis: fermion creation operator is code 10, fermion annihilation operator is code 11...
• Update PauliString class with normalization: sort indices and eliminate combine Pauli operators on the same wire.
• Normalize PauliStrings before exporting to Cirq.
• Added normalization to MajoranaStrings.
• Created helper functions "vertex_operator" and "edge_operator" common in fermionic models.