Add external radix sort and minor fixes

src/ext_sort.{h, cpp}
 - Provide ext_sort funtion for sorting EdgeIntervals in a given filen

Makefile
 - add ext_sort to lsg target

src/search.{h, cpp}
 - Modified step_left and step_right to correctly use external files for
   LT

src/util.{h, cpp}
 - Add operator>> for EdgeInterval* from ifstream

src/lsg/lsg.cpp
 - lsg is now using external radix sort for sorting left-terminal joined
    intervals

src/edge_joined_interval.h
src/interval_manager.h
 - minor code style tweaks