Syngine doesn't work for synthetic inversions

As currently implemented, Syngine doesn't work with synthetic stations. This is because we're providing a URL to Syngine with the station and network info and asking Syngine to look up the coordinates for us (which is a nice feature of Syngine's IRIS FDSN integration).

We can — and should — use receiverlatitude and receiverlongitude options (see Syngine URL docs) to allow synthetic stations which aren't on IRIS FDSN since they're imaginary.