Replace equality comparison with identity comparison for None checks

[Copilot]

Addresses PEP 8 compliance feedback from #92 discussion thread regarding None comparisons.

Changes

• Updated two None comparisons in Snakefile to use identity operator (is None) instead of equality operator (== None)
  • Line 44: stain_for_reg validation
  • Line 55: atlas_segs configuration check

# Before
if stain_for_reg == None:
    raise ValueError(f"a stain for registration was not found in {stains}")

if config["atlas_segs"] == None:
    atlas_segs = all_atlas_segs

# After
if stain_for_reg is None:
    raise ValueError(f"a stain for registration was not found in {stains}")

if config["atlas_segs"] is None:
    atlas_segs = all_atlas_segs

Identity comparison with singleton objects is both more efficient and semantically correct per PEP 8.

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