Porcelain for constructing a Target

[garrison]

Right now the example in the readme of constructing a Target uses some low-level (qk_*) functions from Qiskit.C. It would be even better if we were to provide a proper interface for doing this in the primary module.

[garrison]

Related: the wrapping of the Target properties is a bit incomplete; in particular, some of the properties listed on the following line are not yet implemented:

Qiskit.jl/src/target.jl

Line 110 in dc7ee1f

union(fieldnames(typeof(obj)), (:num_qubits, :num_instructions, :dt, :granularity, :min_length, :pulse_alignment, :acquire_alignment))