remove non-const iteration methods from Vcf::SampleData
make appendFormatField* methods protected in Vcf::SampleData
add isReflect{ed,ion} methods to Vcf::Header

to indicate if and how a sample is involved in sample mirroring
bugfix (rt#97906): copy-on-write when adding filters to mirrored cols

this bug presented as mirrored columns obtaining consensus filters when
the merged columns they are mirroring have said filters applied