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?
They are necessary changes which we should merge, certainly.

I think it's OK to have them as part of this PR, but I think it's confusing for them to be in the same commit as substantive changes.