Added template function select_if() in variant.h; Unit-tested.

I changed some values of GQ and PL in order to test several conditions.  test_variants.vcf has all GQs set to 35 and all genotypes are 0/0, 0/1, 1/1 among the 3 samples.
Since variant_reader_test.cpp already tests is_hom_ref(), is_het() and is_hom_var() on this file, I don't want to modify it.