fix ufm params

[srmnitc]

This pull request updates the run_integration method in calphy/liquid.py to improve flexibility and accuracy in defining simulation parameters. The changes primarily focus on making the ufm (Uhlenbeck-Ford model) parameters configurable through class attributes instead of hardcoding them.

Key changes to simulation parameter configuration:

• Updated the pair_style command to use the ufm_cutoff attribute instead of a hardcoded value [1] [2].
• Modified the pair_coeff command to include the sigma parameter from the uhlenbeck_ford_model attribute, making it more flexible and aligned with the model's configuration.