Warning on main

[RemDelaporteMathurin]

I noticed this warning on main:

/home/remidm/FESTIM/festim/exports/derived_quantities/derived_quantities.py:49: DeprecationWarning: The derived_quantities attribute will be deprecated in a future release, please use festim.DerivedQuantities as a list instead
  warnings.warn(

Even though I'm not using the derived_quantities attribute. I assume it's used somewhere else in the code

[RemDelaporteMathurin]

Found the culprit line:

https://github.com/festim-dev/FESTIM/blob/af31da22cf1fb6ef0b7357b79d1b205b6e7d7a48/festim/generic_simulation.py#L322

Also need to update the documentation:

https://github.com/festim-dev/FESTIM/blob/af31da22cf1fb6ef0b7357b79d1b205b6e7d7a48/docs/source/userguide/export_post_processing.rst#L192