Allow for different CAM backends

[SjoerdOptLand]

To allow for simulation under HFSS, for example, GDSII output would be necessary, which can be generated with GDSPy. If we could conform the RS274X (Gerber) backend to the GDSPy API, the user can interchange both backends.

Dreaming on, we note that Gerber and GDSII are copper image formats, while Excellon is the only drill map format. These files can be bundled together in different ways: in a human-readable directory, or in a formalised ODB++ archive (or IPC-2581). Maybe there should be Stack backends that call upon image backends.