Added CortexGraph.pm; Minor fixes to VCFFile.pm; Added labelled output to sim_compare.pl

Added CortexGraph.pm
- constructs a de bruijn graph in memory

VCFFile.pm:
- Fixed format bug in VCFFile when no samples

sim_compare.pl:
- Added printing labelled truth VCF

sim_vcf.pl:
- Remove extra hash symbol from header (fix typo)