Add dataloaders for variant-effect prediction

[Avsecz]

Implement efficient dataloaders for each dataloaders performing in-silico mutagnesis.

Schema

{
  "inputs" {
       "ref": {},
       "alt": {},
  "metadata": {
     "variant": {}   # TODO - have a standard variant class
      "seq": {"ranges": kipoi.metadata.GenomicRanges   } 
}
}

Features

• support indels (related to Variant effect prediction also for indels kipoi#66)