Add ImageNumber distinction through TableNumber capabilities

[d33bs]

Description

This PR seeks to address #30, providing a way to differentiate between possible ImageNumber overlaps implementing a "TableNumber" value for tables. TableNumber is a checksum based on the source data file and represents a unique value which will help keep CytoTable output distinct.

Other notable changes in this PR:

• Increased modularity, spreading work into multiple files for package and tests to help organize and decrease file-level complexity.
• Added pytest-xdist and default pytest configuration for improved test time performance

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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• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• New and existing unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
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[d33bs]

Closing this in lieu of new requirements and simplified / smaller changes.