fix maxswap in randomize_graph_partial_und

We always returned after first swap regardless the given maxswap argument.
This bug does not exist in upstream matlab BCT project.
randomize_graph_partial_und, minor style fix

This function always want's to write to i and j. No need to optimize
that, just set WRITABLE flag one time at the beginning.
fix return value in randomize_graph_partial_und

This function was unusable until now.
Merge pull request #5 from rudimeier/randomize_graph_partial_und

fix Randomize_graph_partial_und -- thanks!