Adding mendelian error code

Uses map and zip in leiu of loop.
Now assumes biallelic, diploid. In theory extensible to polyploid multiallelic, but too impractical for the moment.

Added test script run with nosetest test/ from the root dir.
Added benchmark script, run with python benchmark/benchmark_ped.py

for 1m variants, runs in approx 6 minutes for 10 samples. The more heterozygotes the slower it runs.