Add read paths speed up
Add read paths speed up

add_read_paths.c: Stop adding paths once you find one already exists
  adding path A,C,G,A: novel -> added
  adding path   C,G,A: already exists stop
  (since GA, A will also already exist)

mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex,mcveanlab/mccortex