remove overload for session launch

[eirrgang]

The launch() member functions are used by a Context implementation when launching a Session. The future plans for this protocol are described in #44 and also relate to #41, #43, #75, #78, #88, #93.

gmx.core.MDSystem.launch() and gmxapi::System::launch() are overloaded to allow automatic generation of a Context object. gmxapi 0.1 / workspec 0.2 will specify that work specifications are managed by a Context chain of responsibility and that all API operations occur within some sort of Context, though a default Context will be created when the API is initialized. This means that a launch() without arguments, if kept, would have its meaning changed from "create a Context" to "use a Context singleton that already exists". For simplicity, I think it is reasonable that launch() should require either a Context or a Session argument. Whether we require one or the other depends on how the session launching protocol is refined, but either way we know we can simplify life sooner than later by removing the overload with the ambiguous signature.