Pauli and Fermion operators add Identity offset when there is a leading +/- sign character

[1tnguyen]

Test cases:

1. Pauli

   TEST(PauliOperatorTester, checkLeadingSignCharacter) {
   std::string src = "- 0.5 Z0 Z1 + 1.2 Z1 Z2";
   PauliOperator op(src);
   std::cout << op.toString() << "\n";
   }

The printout contains an extra +1 offset: (1,0) + (-0.5,-0) Z0 Z1 + (1.2,0) Z1 Z2

2. Fermion

   TEST(FermionOperatorTester, checkLeadingSignCharacter) {
   std::string src = "+ 0.5 1^ 0 + 1.2 2^ 3";
   FermionOperator op(src);
   std::cout << op.toString() << "\n";
   }

   The printout contains an extra +1 offset: (1,0) + (0.5,0) 1^ 0 + (1.2,0) 2^ 3