Inconsistent struct definitions between Swift and Velociraptor in Velociraptor interface

The struct siminfo in velociraptor_interface.c is used to pass information to Velociraptor when it's run on the fly. Velociraptor has a corresponding definition in swiftinterface.h. I believe these should correspond, but they don't.

In Velociraptor the siminfo struct has an extra entry numcellsperdim which doesn't exist in Swift. I think the code only works as it is because we got really lucky with alignment so the next entry in the struct is at the same byte offset whether numcellsperdim exists or not. The other problem is that an option was introduced in Velociraptor that removes the mass field in the particle struct if all particles have the same mass. This requires Swift to tell Velociraptor what the constant mass is, but I can't see any code to do this. There is a siminfo.mass_uniform_box field in Velociraptor but it doesn't exist in Swift.