Stores a numeric value iff this INFO field is a scalar
key: numeric tag id, the corresponding string is bcf_hdr_t::idBCF_DT_ID[$key].key
type: one of BCF_BT_* types; len: vector length, 1 for scalars
type: one of BCF_BT_* types; len: vector length, 1 for scalars
only set if $len==1; for easier access
pointer to data array in bcf1_t->shared.s, excluding the size+type and tag id bytes
length of the vptr block or, when set, of the vptr_mod block, excluding offset
INFO field data (§1.4.1 Fixed fields, (8) INFO)