Simplify/Refactor temperature as a function in Hydrogen problem classes

https://github.com/festim-dev/FESTIM/blob/c6aec5a9f4415a699bf73bcbf4c2426ffddcf064/src/festim/hydrogen_transport_problem.py#L576-L597

This is a mess and needs to be looked at and probably fixed or at least polished.

For example, here we create a new function everytime whereas we should store the Expression somewhere

Start by reading src/festim/hydrogen_transport_problem.py around lines 576-597 and trace how temperature is used in the Hydrogen problem classes. Check where the Expression is created and how its lifetime is managed. Done means the repeated function creation is removed and the temperature Expression is stored appropriately, with existing behavior preserved.

Written by the indexing model from the issue text.