parallel gets for list slices

[MRossol]

Why this feature is necessary:
Even with the slicing tricks implemented here: https://github.com/NREL/rex/blob/ae280dbfc149db95135b25b244c585f09d2d7cb4/rex/resource.py#L133-L445
extracting data from h5py and h5pyd using a list is slow.

A possible solution is:
The most performant solution would be to use concurrent futures to extract each item associated with the list in parallel and then recombine the requests. This would greatly simplify the logic noted above. The only issue is that it wouldn't be compatible with the already parallel nature of reV Generation as you can't run parallel workers on parellel workers.

Additional Context
Relevant for: NatLabRockies/reV#335

Charge code
reV

Urgency / Timeframe
TBD