Add tau_ei to TORAX outputs

[dylantirandaz]

Fixes #2237.

Adds electron-ion collision time tau_ei as a face-grid post-processed output. The value uses the existing Wesson electron collision-time formula scaled by Z_eff, matching the effective electron-ion collision frequency convention used by calc_nu_star.

Tests:

• python3 -m pytest torax/_src/physics/tests/collisions_test.py torax/_src/output_tools/tests/post_processing_test.py torax/_src/output_tools/tests/output_test.py
• python3 -m pyink --check torax/_src/physics/collisions.py torax/_src/physics/tests/collisions_test.py torax/_src/output_tools/post_processing.py torax/_src/output_tools/output.py torax/_src/output_tools/tests/post_processing_test.py torax/_src/output_tools/tests/output_test.py

[jcitrin]

Thank you for your desire to contribute to TORAX. However we have another PR out which provides nu_star output #2203 , which is basically redundant information to tau_ei from the scattering rate.

What may be more interesting is to output the equipartition timescale from coll_exchange, since that timescale as a ratio of the confinement time provides a useful measure of expected ion-electron temperature coupling. However we'd like to do a bit of refactoring first. Closing this PR - we will carry this out. Thanks again.

[dylantirandaz]

No worries, ty