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?
I only needed some way to access the headers of the input samples. I should just add a function get_input_vcf_headers() in MultipleVariantReader.
But, this raises a problem in foghorn for using the LUT. If you are not sure of the order in which variants from different files are returned in the vector, how do you know which LUT entry to look into?