Ensure that VariantBuilderMultiSampleVector works with 0 samples / 0 values per sample

-Make sure we don't crash when creating empty VariantBuilderMultiSampleVectors

-Tests for VariantBuilder already exist confirming that the builder will clear
 the relevant field when given an empty VariantBuilderMultiSampleVector
Merge pull request #394 from broadinstitute/dr_test_multi_sample_vector_with_max_size_zero

Ensure that VariantBuilderMultiSampleVector works with 0 samples / 0 values per sample
Prevent GT from being set using generic integer field setters

Now that genotype encoding has been encapsulated within VariantBuilder,
setting GT using a generic integer setter no longer works.

Resolves #387
Merge pull request #395 from broadinstitute/dr_prevent_gt_field_set_using_integer_setters

Prevent GT from being set using generic integer field setters
Fixed reallocation condition in VariantHeaderMerger