Remove obsolete call in merge_variant_headers()

Now that htslib expects us to sync headers manually, we don't
need to pass in a nullptr when we're done adding samples to
a header

Resolves #363
Merge pull request #364 from broadinstitute/dr_remove_obsolete_call_in_merge_variant_headers

Remove obsolete call in merge_variant_headers()
Change m_pending_variants to a vector
Merge pull request #367 from broadinstitute/jt_pending_vector

Change m_pending_variants to a vector
Prevent vector end values from getting transformed during Genotype encoding, and detect invalid values

Previously we were incorrectly encoding vector end values in Genotype::encode_genotype().
Now vector end values are skipped, and invalid values are detected (only allele indices
>= 0, -1 for missing genotypes, and vector end values are allowed).
operators ++ and -- in IndividualFieldValue iterator should not return
anything. Expecting a return may cause access to out of bounds memory
access.
VI minor changes: const and default
MVR minor changes: add a few moves
RBSVI minor changes: reference, comments, whitespace
Diversify the ref bases for a slightly better test
Merge pull request #370 from broadinstitute/jt_rbs_updates

A few small changes through the Ref Block Spilt stack
Merge pull request #369 from broadinstitute/dr_fix_genotype_encoding_of_vector_ends

Prevent vector end values from getting transformed during Genotype encoding, and detect invalid values
Merge pull request #368 from broadinstitute/intel_individual_field_value_iterator_increment

Void return type for operator ++ and -- in IndividualFieldValueIterator
Revert "Make CMAKE the default build script"

This reverts commit ffe7d3bccf7fe3854d695f9df451ad46c54b31e5.
1. Added MergedVCFLUT capability in utils/merged_vcf_utils.*. This class
is now used within MultipleVariantReader.
2. Added test case for LUT.
3. Fixed signed-unsigned comparisons
4. Added some useful functions in VariantHeader

TODO:
1. More comments for LUT classes
2. What should be done when sample names are duplicated?