more convenient conversion methods

[Krastanov]

It would be nice if QuantumClifford.CliffordOperator(bpgate) and QuantumOptics.Operator(bpgate) worked out of the box. Here is the current workaround

julia> using QuantumOpticsBase, QuantumClifford, BPGates

julia> cnotperm = CNOTPerm(3,4,1,2); # a gate you want converted

julia> accumulate = one(CliffordOperator,2); # an identity operator which we will multiply by the Cliffords that make up cnotperm
X₁ ⟼ + X_
X₂ ⟼ + _X
Z₁ ⟼ + Z_
Z₂ ⟼ + _Z

julia> for gate in BPGates.toQCcircuit(cnotperm) # we are building up the transpiled Clifford from the 
           accumulate = gate*accumulate
       end

julia> accumulate # this is the Clifford gate you want
X₁ ⟼ + Z_
X₂ ⟼ + _Z
Z₁ ⟼ + X_
Z₂ ⟼ + _X

julia> Operator(accumulate) # and here it is converted to a unitary gate representation
Operator(dim=4x4)
  basis: [Spin(1/2) ⊗ Spin(1/2)]
 0.5+0.0im   0.5+0.0im   0.5+0.0im   0.5+0.0im
 0.5+0.0im  -0.5+0.0im   0.5+0.0im  -0.5+0.0im
 0.5+0.0im   0.5+0.0im  -0.5+0.0im  -0.5+0.0im
 0.5+0.0im  -0.5+0.0im  -0.5+0.0im   0.5+0.0im