pull available_fields and field_descriptions of VafInterpreter into helper class
pull available_fields and field_descriptions of ManySamplesVafInterpreter into helper class
pull single_vaf_headers and per_library_vaf_headers of FullReporter into helper class
get basic_available_fields from single_vaf_fields and per_library_vaf_fields to remove code duplication
Merge pull request #148 from susannasiebert/vaf_fields

Make a helper module to consolidate VAF interpreter fields