Some refactored statements need recheck

[HNYuuu]

I am refactoring the graph.py, some of the statements need recheck @zzhzz

• emulator.py
  • Line 159, can we just remove the halt?
• graph.py
  • Line 428, the function calc_circle has initialized vis, but at line 418 the vis is re-initialized;
  • Line 466, I have revised the behavior of initialization of branches to make it inline with the data structure at line 470, please recheck it;
  • Line 521, please recheck if the return statement is correct when the specify_branch is enabled or not;
  • Line 534, do we still need function extract_edges?

Please recheck this comments, and revise the corresponding statements. Thanks @zzhzz

[zzhzz]

• emulator.py : halt can be removed, since the returned value in L600 of graph.py will be ignored.
• graph.y:
  • vis may be modified by the invocation of calc_circle at L426.
  • NP.
  • DFS-way traversal still remains some bugs, the refactor of visit can be postponed.
  • extract_edges will be used in debugging, maybe we can move all debugging-related code into one class, and use descriptor to debug functions

[HNYuuu]

The first three problems are revised in ae4ac2d, 829b635 and 1dc1715, respectively.

However, the DFS-traversal algorithm seems buggy, please refer to #39; and the debugging information and functions should be extracted, please refer to #40.