Add low level methods for handle graphs

[rlorigro]

I've been using paths quite a lot in bdsg and found that a couple methods would simplify things:

pop_path_front/back
rename_path

Based on the existing methods prepend_step and append_step, pop should be equivalently complex for each implementation. As it is now I am using this to "pop" elements:

        auto begin = graph.path_begin(p);
        auto next = graph.get_next_step(begin);
        graph.rewrite_segment(begin, next, vector<handle_t>());

And for the path renaming, I copy the entire path and delete the old one, so it would be cool if there was a copy-free method.

[rlorigro]

A couple more that may be worth adding:

flip_edge(edge_t)
get_edge_count(handle_t h, bool go_left)   # nevermind about this one, just learned it is called get_degree()

[rlorigro]

Another method that might be nice:

for_base_in_sequence(const function<void(char& base)>& f)

At the moment, the only way to iterate every base in a sequence (for kmer counting, for example) is to copy the whole sequence, or to use get_base() which may have to use a hash map for every iteration, depending on which implementation is used.