multicloud: update the cytomining workflow to have multiple tasks

[deflaux]

As part of one possible approach to enable #41, update cytomining.wdl to have multiple tasks.

Specifically, call task is_bucket_writable (renamed from gcloud_is_bucket_writable) at the beginning of the cytomining workflow and call task extract_and_copy (renamed from extract_and_gsutil_rsync) at the end so that it can take advantage of multi-cloud support.

(Note that cytomining_jumpcp.wdl is a bit more complicated, as it has support to perform federated auth to AWS when running in GCP so that inputs can be read from S3 and outputs can be written to S3.)