Fix opachord.py to avoid nan in the gradient of ArtTransPure model

[sh-tada]

This pull request resolves the issue (#463) of NaN values in the gradient of the ArtTransPure transmission spectra model with respect to temperature, gravity_btm, radius_btm, and mean molecular weight. The NaN values originated from the jnp.sqrt function in exojax/spec/opachord.py.

To address this, I implemented jnp.tril before the jnp.sqrt operation. This modification zeros out irrelevant values, ensuring proper gradient calculations. Setting k=-1 in jnp.tril also zeroes the diagonal.

Fixes #463

[HajimeKawahara]

Can you add a test file shown in #463?
maybe in integration/unittest_long/transmission/

[sh-tada]

I added transmission_grad_test.py in integration/unittest_long/transmission/.
If necessary, I will change the file name.

[HajimeKawahara]

The newly added file is not pytest file. maybe remove _test from the filename?

[sh-tada]

I modified the filename and Nx.

[HajimeKawahara]

Thank you!