SparseSimulator does not support qudit operations

[smitsanghavi]

SparseSimulator code assumes a system of qubits only and does not support general qudits.

Running the simple Circuit below gives a ValueError size 2 is different from 3.

qudit = cirq.NamedQid("a", 3)
circuit = cirq.Circuit(qudit_gates.QuditXGate(3).on(qudit), cirq.measure(qudit))
SparseSimulator().run(circuit).measurements

[smitsanghavi]

#76 partially gets around this since there are no qudits in the circuit after it gets compiled to qubits, so SparseSimulator can be used on a qudit-based QuantumWorld that has compile_to_qubits=True. We still need to fix the SparseSimulator for when compile_to_qubits=False (default)