[vcf_etl] Property `length` from header.Coting object must be integer

[dgomezpere]

Detected that Contig.lenght is str instead int

> db.vcf_contigs.find()
{ "_id" : ObjectId("61a60a1f9b783171e22dcf31"), "id" : "chr1", "length" : "248956422" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf32"), "id" : "chr2", "length" : "242193529" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf33"), "id" : "chr3", "length" : "198295559" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf34"), "id" : "chr4", "length" : "190214555" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf35"), "id" : "chr5", "length" : "181538259" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf36"), "id" : "chr6", "length" : "170805979" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf37"), "id" : "chr7", "length" : "159345973" }
{ "_id" : ObjectId("61a60a1f9b783171e22dcf38"), "id" : "chr8", "length" : "145138636" }