[BugFix] Removed array initialisation in Mesh constructor

Changes from a recent PR did not compile on Windows as array initialisation in initialisation lists is not allowed in MSVS (see here: http://msdn.microsoft.com/en-us/library/9f53ks1w.aspx)
I replaced the ```_node_distance``` and ```_edge_length``` arrays with pairs. These *can* be initialised in the init lists and imo make more sense than an array of length 2 anyway.
I hope this is okay for everyone.