Use prep_batches in gcp.py

[nweires]

Updates the GCP implementation to share code with AWS for setting up batches of simulations to be run in parallel.

Testing: I did a test run to check that everything still works.

[github-actions]

File	Coverage
All files	86%	✅
base.py	90%	✅
eagle.py	77%	✅
exc.py	57%	✅
local.py	70%	✅
postprocessing.py	84%	✅
utils.py	91%	✅
cloud/docker_base.py	93%	✅
sampler/base.py	79%	✅
sampler/downselect.py	33%	✅
sampler/precomputed.py	93%	✅
sampler/residential_quota.py	61%	✅
test/test_docker.py	33%	✅
test/test_validation.py	97%	✅
workflow_generator/base.py	90%	✅
workflow_generator/commercial.py	53%	✅
workflow_generator/residential_hpxml.py	86%	✅

Minimum allowed coverage is 33%

Generated by 🐒 cobertura-action against d95a38e

[nweires]

@lathanh This may all be covered by your changes in #34, in which case I can just close this PR. But if it makes the changes there more concise/clear, I can merge this first. Either way is fine with me.