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?
(should also update the documentation at the top of the class to reflect the difference between get_merged_header() and get_raw_merged_header() after making this change)