Factor out small community detection

[ahill187]

Description

In this PR, I have factored out the code that merges small communities. There was a lot of duplicated code between fit_predict and run_toobig_subPARC, so this addressed that issue.

I have created three new functions:

1. small_community_merging: This function runs the merging functionality, with help from the other two functions.
2. get_small_communities: A helper function to find which communities are too small.
3. reassign_small_communities: A helper function which iteratively reassigns the small communities to other communities.

I have renamed several of the variables:

original	new	reason
cluster	community_id	For consistency, in keeping with graph theory and Leiden algorithm definitions
population	community_size	population is ambiguous, it could be a single value or a set
small_cluster	small_community_indices	For consistency, in keeping with graph theory and Leiden algorithm definitions
single_cell	sample_id	PARC is not only used for cells; it can take any type of data
old_neighbors	neighbors	Using old as a modifier is poor practice
group_of_old_neighbors	node_communities_neighbors	This is the same data type as node_communities
best_group	community_id	Group implies a set, but this is a single ID

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (updated docstrings or README files)
• Tests (added new tests or modified current test suite)
• Refactoring (changes in the design of the code that don't change the functionality)

Tests

• tests/test_parc.py: test_parc_get_small_communities
• tests/test_parc.py: test_parc_reassign_small_communities
• tests/test_parc.py: test_parc_small_community_merging

Linked PRs / Issues

ahill187#29

Checklist:

• I have performed a self-review of my code
• I have documented my code with appropriate docstrings
• I have made corresponding changes to the documentation / README files
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes