FeatureB_9 by Group 19

[Swar-16]

This PR introduces set operation support by adding:

• set.py
• test_set.py

No changes were made to calculator.py in this PR.

What Was Added

• Implemented set-related functionality in set.py.
• Added tests in test_set.py to validate set operations and expected behavior.

What Was Not Changed

• calculator.py was intentionally left unchanged due to merge/conflict risk.
• Different groups are using different conventions for implementing modes in the Calculator class and for wiring set operations through that class.
• To avoid introducing convention-specific conflicts (setting up mode), this PR scopes changes to new set module + tests only.
• We intend to handle integration of set operations into Calculator modes in a follow-up PR once discussed in class.

Testing

• Added and executed tests for set functionality in test_set.py.
• Existing calculator behavior remains unaffected since calculator.py was not modified.

Set Functionality

• Implemented basic set operations as:
  • Union
  • Intersection
  • Difference
  • Symmetric Difference
• Also, subset and superset checks were implemented.