Fix unguarded gudhi.tensorflow import crashing data_assimilation on plain install (#197)

[OmerJauhar]

Description

data_assimilation.py had from gudhi.tensorflow import RipsLayer at the top of the file (line 3), outside any try/except block. Since gudhi.tensorflow internally imports TensorFlow, this caused the entire module to crash on import when TensorFlow was not installed, even though TensorFlow is an optional dependency only needed by TADA.The fix moves the import inside the existing try/except block and replaces the bare except with except ImportError. Instead of raising immediately at module load time, a flag _tensorflow_available is set and checked at the start of TADA. This means:

• rafda can be imported and used on a plain pip install teaspoon with no TensorFlow
• Calling TADA without TensorFlow raises a clear ImportError pointing the user to pip install 'teaspoon[full]'

Motivation and Context

Closes #197. " teaspoon.DAF.data_assimilation crashes on import after plain "pip install teaspoon" => unguarded from gudhi.tensorflow import RipsLayer on line 3 defeats the developer's own tensorflow try/except #197 "

How has this been tested?

• Confirmed from teaspoon.DAF.data_assimilation import rafda succeeds without TensorFlow installed.
• Confirmed from teaspoon.DAF.data_assimilation import TADA succeeds without TensorFlow installed.
• Confirmed calling TADA(...) without TensorFlow raises ImportError with the message "TADA requires TensorFlow. Install with: pip install 'teaspoon[full]'".
• Verified via AST check that RipsLayer is no longer imported at the module top level.
• Existing DAF test suite (tests/test_DAF.py) — 1 skipped (expected: the test correctly skips itself when TensorFlow is not available).

Types of changes

• 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)

Checklist

• My code follows the code style of this project. (make clean)
• My change requires a change to the documentation.
• All new and existing tests passed. (make tests)