add shortest 'module' to keep track of shortest paths, add calc:clean()
command to clean the state of unneeded paths

usage

application:start().
calc:reload_data("data1.dat").
calc:reload_msg().
calc:clean(), calc:compute(), calc:next_step().
calc:clean(), calc:compute(), calc:next_step().
calc:clean(), calc:compute(), calc:next_step().
...
%% check ets for shortest paths