Reorganize Temperature Conversion Functions and Constants

There are some existing [F to K temperature conversion functions](https://github.com/google/sbsim/blob/copybara_push/smart_control/utils/conversion_utils.py#L137-L170) in "smart_control.utils.conversion_utils.py".

In the new reinforcement learning module, we added some C to K conversion functions in "smart_control.reinforcement_learning.utils.data_processing.py" :

https://github.com/google/sbsim/blob/46e01d97f4ced83fb8f5d91be7d351e45d3fd3fd/smart_control/reinforcement_learning/utils/data_processing.py#L323-L348

We should consider moving them up into the more general location ("smart_control.utils.conversion_utils.py").

https://github.com/google/sbsim/pull/19#discussion_r2040261783_