[FORMATS-138] Rename NucleotideContigFragment start/end fields.
[FORMATS-138] Rename NucleotideContigFragment start/end fields.

Resolves #138. Makes NucleotideContigFragment start and end fields consistent
with all other record types that map to a single genomic range.

heuermh/bdg-formats,bigdatagenomics/bdg-formats