Added LUT for merged header in MultipleVariantReader

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. What should be done when sample names are duplicated?
Your VariantHeaderMerger, as part of gamgee, should really accept gamgee VariantHeaders instead of htslib bcf_hdr_t pointers. This would also allow you to leverage the existing VariantHeader functions to query things like field existence instead of having to make htslib calls everywhere.