fixes #77

[pgawron]

No description provided.

[adamglos92]

Proposed changes fixes the UnitaryChannel case, but still does not fix general case because of the code below.

function compose(Φ1::T, Φ2::T) where {T<:AbstractQuantumOperation{<:Number}}
    compose(T, Φ1, Φ2)
end

Furthermore I still suggest to implement separate functions for various combination of composition of IdentityChanneland UnitaryChannel
I think I can propose some additional tests

id = IdentityChannel(2)
u1 = UnitaryChannel([cos(1) sin(1); -sin(1) cos(1)])
@test compose(u1, id)
@test compose(id, u1)
@test compose(id, id)

Φ = KrausOperators([[1 0; 0 0], [0 0; 0 1]])
Φsparse = KrausOperators([sparse([1 0; 0 0]), [0 0; 0 1]])
@test compose(Φ, Φ)
@test compose(Φ, Φsparse)

Not sure what behavior should be expected if Matrix types differs, taking into account the fixes for Unitary case both should pass.

[lpawela]

@adamglos92 sparse matrices are currently NOT supported, use at your own risk.