Add support for parsing operator expressions in op

[mtfishman]

For example:

julia> using QuantumOperatorDefinitions

julia> op("X + 2Z")
2×2 Matrix{Float64}:
 2.0   1.0
 1.0  -2.0

julia> op("exp(im * (X + 2Z))")
2×2 Matrix{ComplexF64}:
   -0.617273+0.70369im   -2.77556e-17+0.351845im
 5.55112e-17+0.351845im     -0.617273-0.70369im

Closes #11 .