Refactor of embedded operators

[andgoldschmidt]

Closes #113

Embedded operators interface makes sure that a user is only ever passing subspace operators,

U_goal = EmbeddedOperator(
    GATES[:CZ],
    system,
    subspace=get_subspace_indices([1:2, 1:2], [3, 3])
)

The purpose of this PR is to reduce the amount of code in embedded operators to essential features.

NOTE: Composite systems do not have test cases; hence, while expected behavior shouldn't have changed, there aren't any guarantees, currently.

[aarontrowbridge]

We're definitely going to want to refactor this at some point to utilize Qobjs from QuantumToolbox.jl. But for now, whatever is easier is best.