VCF/BCF core lib: Allow missing values in vectors. Note that this is a major change which, in contrast to BCFv2.1 specification, allows missing values in vectors. For integer types, the values 0x80, 0x8000, 0x80000000 are interpreted as missing values and 0x81, 0x8001, 0x80000001 as end-of-vector indicators.  Similarly for floats, the value of 0x7F800001 is interpreted as a missing value and 0x7F800002 as an end-of-vector indicator. This trial BCF version (v2.2) is compatible with the VCF specification and enables to handle correctly vectors with different ploidy in presence of missing values.