make MC updates independent from the diagram topology

[kunyuan]

Calculating the weight of a diagram only need to know the internal variables and the G and interaction functions. It does not rely on how G and interaction are topologically connected. Such observation can be used to make the code as generic as possible.

The following may be useful in the future:

1. Each group in the diagram file should provide a list of independent variables, with external and internal variables separately marked. There is no need to provide diagram topology in principle.
2. In the code, each group object maintains the same list of independent variables, and a list of related G and interactions.
3. MC updates simply use these variables to calculate the weight of G and interaction.