feat: Implemented support for MultiMacenkoNormalizer across all backends

[andreped]

fixes #57.

Changes

• [x] Implement support for TensorFlow
• [x] Implement support for NumPy
• [x] Added new tests for checking that different backends produce the same result
• [x] Minor refactoring

Testing

To test code, you can either run directly or do what I do in the tests. See these examples:

• https://github.com/andreped/torchstain/blob/feat/multitarget-macenko-support-all-backends/tests/test_torch.py#L131
• https://github.com/andreped/torchstain/blob/feat/multitarget-macenko-support-all-backends/tests/test_tf.py#L84