[ENH] Improve `get_chromosomes_from_ensebml` code and logic

[fraimondo]

In get_chromosomes_from_ensebml https://github.com/juaml/hipsnp/blob/76963bfa47164baa2afaf41c55a0b72763c36578/hipsnp/hipsnp.py#L634

Problem: We currently do one request per RSID and parse the json in a not-so-good way.

We can improve this method by:

1- Using a combined request to ensembl.org REST API. 2- Checking the json response properly.

Todo:

• [ ] Replace the GET request for each RSID to variation/human/ (https://rest.ensembl.org/documentation/info/variation_id) for a POST request to (https://rest.ensembl.org/documentation/info/variation_post)
• [ ] Parse the json file and check properly the mappings list. We need to decide what to do if we find more than one. Currently, we just take the first.