I had to wrap LinearTransform.operator() in afw.
Otherwise it was straightforward.

However, I am not very happy with astrometry_net.cc. It has a lot of C++ code mixed in with the usual pybind11 wrapper. The extra code is there as a shim to astrometry_net's C code, so it's reasonable to have there, but the file feels cluttered to me. Suggestions for cleanup welcome.

Note that we will be moving our wrapper of astrometry_net into a new package at some point (soon after we switch to pybind11, I hope, but so far other things have always taken priority). So if a very invasive sort of cleanup is wanted, it might be best to defer that until the split.

Looks good.

I would however prefer to split the C++ shim from the wrapper code.
Feel free to disagree, or choose between a single `astronomy_net.h + .cc` combo (in the standard `include` / `src` locations) or having separate `MultiIndex.h` + `Solver.h` (+ `.cc`) files.

I split the file as you suggested and added MultiIndex.__iter__ in Python. Note that I left the logging functions in the pybind11 wrapper because I think they would need more work to be made visible to C++ code (more than I think is justified at this time).

Please have another look.

Looks good.