Replace _pool with _qubit_pool

[JonathonMisiewicz]

At present, the Algorithm class uses a _pool_ attribute, which represents a linear combination of fermionic second quantized operators. This is inappropriate for systems that are not fermions, or for algorithms with an operator pool for which expressing things in terms of second quantization is inconvenient. These should be replaced with an operator pool expressed solely in terms of qubit-space operators.