Move stopping conditions from KWNModel to GenericModel

[nury12n]

Stopping conditions are currently only available for KWNModel, but I would like to have it more general to be part of GenericModel. This would allow for stopping conditions to be used for the diffusion models as well.

Additional stopping conditions that can be added afterwards could include:

• Simulation time to produce a hard stop if a simulation is taking too long
• Metric to indicate if a composition profile is near homogeneous, which may be used to determine how long it would take to homogenize a casting