Use CloudOS and NextFlow to integrate TCGA and GTEx data

See [GTEX_TCGA_Federated_Analysis notebook](https://github.com/ga4gh/fasp-scripts/blob/3f4019d347f7611aab8bd4322b32add5c51b82c6/notebooks/GTEX_TCGA_Federated_Analysis.ipynb) for an iPython workflow.

The overall flow of the script/notebook is probably more illustrative that directly usable in NextFlow. A good question is how to do the equivalent work from CloudOS and NextFlow where that is the platform of preference e.g. as it is for the JAX team. If it is of interest to you to explore that we could work together to get started.

I have no experience of writing NextFlow, I’ll leave that to you. Nevertheless, I was able to get a sense from the nf script that Sangram had shared previously of how you structure things.
https://github.com/lifebit-ai/sra-dbgap-datafetch/blob/main/main.nf

One of the questions is which capabilities it makes sense to do from with NextFlow, and where those capabilities should be in some library which NextFlow calls. Either way, I think we could do some useful experimentation. Some of the fasp package may be useful, if not I could modify it to help.

See #15 for a possible additional step.