Bug fix

[srmnitc]

This pull request refines the handling of thermodynamic integration and simplifies the initialization logic for equilibration control in the calphy package. The most important changes include fixing a reference to the sigma parameter in the thermodynamic integration method and removing redundant conditional logic for setting the equilibration control.

Fixes and improvements in thermodynamic integration:

• In calphy/liquid.py, corrected the reference to the sigma parameter by ensuring it is accessed from self.calc.uhlenbeck_ford_model instead of self.uhlenbeck_ford_model. This ensures consistency and prevents potential attribute errors.

Simplification of equilibration control logic:

• In calphy/phase.py, removed redundant conditional checks for the reference phase when setting the equilibration control. The logic now directly sets the equilibration_control to "nose-hoover" without phase-specific conditions, simplifying the code.