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?
If you take my advice to refactor the header merger API to accept VariantHeaders, these calls can all become calls into the appropriate VariantHeader functions (has_filter(), has_shared_field(), has_individual_field(), etc.)