convert batch_size into a package constant

[StevenKauwe]

Citrine Python PR

Description

The current hypothesis is that sometimes the objects we send to datasvc are so big that they cause a time-out during writing. @kenneth Kroenlein One workaround for this is to reduce the batch-size configuration in citrine-python/gemd_resource.py from 50 to a lower value.
https://citrine.atlassian.net/browse/PLA-10156

To address the user pain point, we should change batch_size = 50 to a package constant BATCH_SIZE = 50 so that users can handle this case without editing citrine-python source code.

PR Type:

• Breaking change (fix or feature that would cause existing functionality to change)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Maintenance (non-breaking change to assist developers)

Adherence to team decisions

• I have added tests for 100% coverage
• I have written Numpy-style docstrings for every method and class.
• I have communicated the downstream consequences of the PR to others.
• I have bumped the version in __version__.py

[kroenlein]

LGTM!

Normally, I'd expect at least one new test with any PR, but this refactor is straightforward and doesn't really change any public functionality.