Template Specialization

* tried explicit template instantiation. Seems to work fine, we should maybe do that throughout the code

* Learned a lot about C++. A templated function can't have partial specialization, nor can't it have default template arguments. A workaround is just to do partial specialization on a class that can be used as a function

* finished debugging tree partition. It is now able to produce the correct result in a simple test case

* BGL: be very careful about add_edge or remove_edge, it invalidates iterators

* Made make_random_graph truly generic

* cleaned up in the code (minor, mainly converting i++ to ++i)

-> continue to test tree partition on more complex cases, clean up code and comments

-> continue to work on genericity

-> reorganize current files, continue clean-up, maybe try to do explicit instantation everywhere