Refactor ColumnVEPField to be simpler data structures

[davmlaw]

Now that we have T2T the combinatorial explosion of different builds is starting to bite

Instead of creating multiple entries in the DB, we could have eg genome_build = [ "GRCh37", "GRCh38"] etc - can do more with JSON than

This is not the final form, but useful to see how much you can remove

records = ColumnVEPField.objects.all().order_by("variant_grid_column").values()

cleaned_data = defaultdict(list)
for r in records:
    data = {k:v for k,v in r.items() if v is not None}
    del data["id"]
    if data.get("source_field_has_custom_prefix") is False:
        del data["source_field_has_custom_prefix"]
    del data["column"]
    vgc = data.pop("variant_grid_column_id")
    data["variant_grid_column"] = vgc
    sf = data.pop("source_field")

    cleaned_data[sf].append(data)

A few strange things:

Mastermind_counts is processed 3 times (and stored in different fields)
gnomAD_SV_AF is copied to both gnomad_sv_overlap_af and gnomad_sv_overlap_af