CalcQuregDerivs invalid on density-matrices

[TysonRayJones]

For state-vectors, CalcQuregDerivs computes

d/dx ( U(x) U(y) |z> )  = dU(x)/dx U(y) |z>

For density-matrices, it makes sense that this function should perform

d/dx ( U(x) U(y) |z><z| U(y)^ U(x)^ ) 

= dU(x)/dx U(y) |z><z| U(y)^ U(x)^ + U(x) U(y) |z><z| U(y)^ dU(x)^/dx

However, currently this function will effect on density-matrices

U(x) U(y) |z><z| U(y)^ U(x)^   ->    c dU(x)dx U(y) |z><z| U(y)^ (dU(x)/dx)^

where c is an incorrect normalisation.

• In the meantime, disable CalcQuregDerivs for density matrices.
• Update CalcQuregDerivs to calculate the correct derivatives of the channel.

[TysonRayJones]

CalcQuregDerivs disabled on density matrices in 5bcdae2