Fix VCF position int overflow (long long) & fix args.ploidy typo (arggs -> args)

[wnsplim]

Two small fixes:

1. VCF position integer overflow: Sampler.cpp reads the VCF POS field into an int, which results in overflow with (simulated) VCFs with a length longer than ~2.1Gb. Solution: simply swap int to long long
2. Typo in singer_master: arggs.ploidy -> args.ploidy.