Improve Memory Performance Within merge_single_cells

[d33bs]

Description

This is a change to enhance performance as per #195 . Pandas rename's with copy=False, inplace=False can sometimes see increased performance, as per this reference.

I have not added a test for this as the change is focused on performance that may be variable contingent on the source data and environment involved (memory capacity may fluctuate).

Please let me know if I may make any changes or if you have any questions/concerns. Thank you!

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

Checklist

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• New and existing unit tests pass locally with my changes.
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