Extend Pauli strings in NoiseConfig to include loss

[orpuente-MS]

Previously you could specify correlated Pauli noise in the NoiseConfig object as follows:

noise = NoiseConfig()
noise.cz.ZX = 0.01
noise.cz.IX = 0.01

With this change, you can also use the L character to specify correlated noise together with the Pauli terms:

noise = NoiseConfig()
noise.cz.LX = 0.01
noise.cz.IL = 0.02
noise.cz.LL = 0.03