Short-circut Annotations

The PR adds the (awesome) ability for state variables to optionally provide their own code snippets - rather than relying on code generation - for all of the pragmas where it makes sense.  Notably, it does not make sense for clone and annotations. 

For example, This allows module developers to provide their own schema while still getting all of the other benefits of the pre-processor. 

```C++
#pragma cyclus var {'schema': '<my>rng code here</my>'}
int mystate;
```

None of the current agents use this functionality, but I see it as highly desirable.