Add MMD unit tests

[jaime-cespedes-sisniega]

This pull request includes updates to the CI configuration and significant enhancements to the unit tests for the MMD data drift detector. The most important changes include updating the macOS version in the CI workflow and adding comprehensive tests for various chunk sizes and kernels in the test_mmd.py file.

CI Configuration Update:

• .github/workflows/ci.yml: Updated the macOS version from macos-12 to macos-14 to ensure compatibility with newer macOS environments.

Enhancements to Unit Tests:

• frouros/tests/unit/detectors/data_drift/batch/distance_based/test_mmd.py: Added constants RANDOM_SEED and DEFAULT_SIGMA for better maintainability and replaced hardcoded values with these constants in existing tests. [1] [2] [3]
• frouros/tests/unit/detectors/data_drift/batch/distance_based/test_mmd.py: Introduced new tests to check the equivalence of MMD results with different chunk sizes and to verify the validity of chunk sizes and kernels.