Add a task for retrieving GEO platform metadata

[arteymix]

We retrieve GEO platform metadata to detect the appropriate steps to use for processing sequencing data.

The step that retrieves the metadata should be a Luigi task that is retried in case of failure.

[arteymix]

In #101, we instead rely on the assay type experiment tag which is parsed from GEO and adjusted as needed by the curation team.