fix: jvp in cfd Tesseract should only return derivative (#12)

#### Relevant issue or PR
Fixes small deviation from jax recipe introduced in
#10

#### Description of changes
Even though it is not used in the cfd demo, `jvp_jit` should follow the
jax recipe and only return the derivative.

#### Testing done
Manual: Tesseract still builds and notebook still runs after changes

#### License

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license](https://pasteurlabs.github.io/tesseract-jax/LICENSE).
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and email address to the `Signed-off-by` line.

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<summary><b>Developer Certificate of Origin</b></summary>

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</details>

Signed-off-by: Heiko Zimmermann [email protected]

Author: zmheiko

demo/cfd/cfd-tesseract/tesseract_api.py

@@ -204,7 +204,7 @@ def jvp_jit(
         filtered_apply,
         [flatten_with_paths(inputs, include_paths=jvp_inputs)],
         [tangent_vector],
-    )
+    )[1]
 
 
 @eqx.filter_jit