SB field Number is changed from 4 to 1 during conversion

[tomwhite]

Found while working on sgkit-dev/vcztools#198

$ bcftools view --header-only ../vcztools/tests/data/vcf/chr22.vcf.gz | grep SB
##FORMAT=<ID=SB,Number=4,Type=Integer,Description="Per-sample component statistics which comprise the Fisher's Exact Test to detect strand bias.">
$ vcf2zarr convert --force ../vcztools/tests/data/vcf/chr22.vcf.gz chr22.vcz
$ vcztools view --header-only chr22.vcz | grep SB 
##FORMAT=<ID=SB,Number=1,Type=Integer,Description="Per-sample component statistics which comprise the Fisher's Exact Test to detect strand bias.">

[jeromekelleher]

It's not obvious whether this is coming from vcf2zarr or from vcztools here. However:

 cat chr22.vcz/call_SB/.zarray 
{
    "chunks": [
        1000,
        10000
    ],
    "compressor": {
        "blocksize": 0,
        "clevel": 7,
        "cname": "zstd",
        "id": "blosc",
        "shuffle": 0
    },
    "dimension_separator": "/",
    "dtype": "|i1",
    "fill_value": null,
    "filters": null,
    "order": "C",
    "shape": [
        100,
        100
    ],
    "zarr_format": 2
}

shows the 3rd dimension indeed 1 and the bug is here.

[tomwhite]

Fixed in #358