brain2reg implementation

[jefferis]

For the time being brain2reg will isolate any path dependencies and so you can choose whatever approach you like and we will not need to change much. It might be a bit more modular if it relied on the object to have a function that returned appropriate paths. Bridging registrations are trickier though. Here we might eventually need another registry style approach (see nat::fileformats) i.e. an environment in nat that stores registrations it know about. Data packages would then use a nat function to tell it about the type location of registrations.