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?
Could we please rename this function to get_raw_merged_header(), and add a new function called get_merged_header() that returns the merged header as a gamgee VariantHeader?